Soundstage drastically worsened when I replaced a solid state AV amp with tubes.

Switch back to the previous setup, but keep the extension cord.  The effect won't necessarily be the same for the two amps, but it might give you an idea of how much it could be impacting things.

Can’t touch this. Except maybe a bumped or stuck mono switch. Is there an indicator?

Thanks!  Good idea.

I agree to remove the extension cord to get back to your baseline. 

Once you are all set using both amps, you may need to adjust room treatments (if using them) for optimal soundstaging.

amps warmed up ?

give time, your ears need get accustomed to new amp.

happened to me

I've found things as small as a change of record weight/clamp on my TT to affect the overall presentation. Readjusting crossovers, etc made a big difference. You might find that repositioning your speakers with the tube amp makes a big difference. I agree about checking the effect of the extension cord. 

All good ideas.  Thanks!

does anyone have any ideas regarding why the soundstage so drastically changed?

Simple; you’re now using a totally different amplifier design (tubes). Welcome to the world of audio equipment. The amp, if brand new, may require a burn-in period. Not all tube amps will sound better than SS, and vice versa. Many tube amp enthusiasts also enjoy tube-rolling to fine tune the sound of their tube amp. How long was the tube amp powered up before you made this analysis? The list goes on and on.

Maybe the Mac 275 doesn't like the McIntosh MX123 A/V processor?

The components are behind the wall in the photo

What photo?

It is as though I am back in the 1950s listening to a single mono speaker directly in front of my listening position.

Are you 100% sure the mode switch on the MC275 is set to stereo and not mono?

Return the 275 and call it a lesson learned.

@therandyman While it does not help much in your case already owning the MX123 pre-pro, a group helping another forum member years back with in a hybrid audio / video system were using the MC2500 preamp (with HT pass-thru) paired with the MC275 tube amp for the 2ch setup, also using a separate AVR to power the rest of the speakers for the theater setup. They were going for a better synergy for the 2ch part of they system if I recall correctly. 

Someone hinted at something earlier... 1) how old is the 275 of yours, 2) how many ours on it?  3) how long are you letting it warm up before listening? ...Is it a brand-spanken-new 275 amp?

Are you 100% sure the mode switch on the MC275 is set to stereo and not mono?

It sure sounds like OP is inadvertently getting mono output, whether by erroneous switch setting or miswiring (even internally - wonder if the amp has been worked on?).

I recently switched from a SS amplifier (the excellent Benchmark AHB2) to tubes (Rogue Stereo 100 Dark) and had the opposite experience—significantly wider and deeper soundstage, but only modest change, if any, to vertical dispersion. The real change, however, is that the SQ has improved in terms of realism—it seems more “organic” and less dry.  As others have suggested, the MC275 (which I considered before buying the Stereo 100) could have a fault if the SQ has collapsed to seem mono.

Wouldn't the Crosstalk specification of different amplifiers, preamplifiers and source affect soundstage?

If the MC275 is new, you need to give it some time. I experienced the same thing with a new solid state amp, it took a couple weeks of continuous play to establish the soundstage. 

More great comments, thanks.  To answer some questions:  The amp was purchased as new, but had been on the dealer's floor as a demo.  The amp itself is well burned-in.  The tubes are new, but have been left "on" for a few weeks prior to installation at my home.  They probably need another 50 or so hours to properly burn in.  There is no photo because I did not know how to properly attach it to my post.  I copied it, pasted it, it appeared on my pre-posting screen, but evaporated when I entered the post.  I also realized that I have different balanced cables running from the amp to the processor.  I'll look into that as well as the power cord.  Thanks again for all the thoughtful, non-judgmental responses!

One more thing:  Yes, the switch is on STEREO.  I had the same thought.  Thanks.

    Recheck (possibly try reversing) phase on the MC 275/LaScala connections.

                 L/R cable (speaker or interconnect) orientation, perhaps?

                           +1, regarding time for amp to break in.

                  Can you try your original processor to amp cable again? 

Beyond all the recommendations already made, you might just consider that the switch from solid state to tubes is the reason you are experiencing the changes you noticed.  I have a McIntosh MC7300 (solid state), and two preamps, the C2200 (tube) and a C38 (solid state).  Although the situation is reversed in my case, with the amp being solid state and the preamps being either tube or solid state, the difference in sound assessment is equally notable.  And not in a bad way at all, just that using either the tube or solid state preamp with the solid state amp leads to a different perception of the music presentation.  And frankly I enjoy both.  

Is it possible your av processor settings changed, or you got the speaker polarity wring before or after?

Meaning, is it possible you had a speaker polarity flipped before, but fixed it now or vice versa??

@therandyman as for the brand-new tubes, yes give them more play time to settle in as you know, and you also mentioned realizing "having different balanced cables". What is the difference if you can share more...

Are these different or the exact same cables you had with the pre-pro processor setup with the solid state amp before?

The cables are different.  The original installation on the MC8207 had Kimber Kables (silver), only a few inches long, to connect the amp to the processor.  The new installation of the 275 is outside the equipment rack, several feet from the processor.  I am not sure of the brand of balanced cable, as it is only temporary, but I'm sure it is a much lower quality than the silver Kimbers.  I'm thinking that is a part of the problem, but no way to tell just yet.  Thanks for the comment.

@therandyman yep, if its a radically different cable design it can surely make a difference, for starters. If they show up, ignore the coat-hanger cable trolls here. Maybe get some loaners from the TheCableCo to try before deciding. Best of luck.

Any chance that your connecting cables aren't a great match for the new amp?  Easy enough to experiment a bit.

negative feedback causes micro transients to be subtly altered and smeared. this can give false cues as to soundstage and detail information. like the exaggerated beauty of breast implants collagen lips and oversized butts. that ain't beauty, it's exaggeration for those with blunt senses. (this is why the best in mediocre audio gear is confused by the audiophile masses as being 'the best', when it is clearly not and those middle quality 'imagined as the best'  brands make all the money, like the fake beauty with the breast implants. the best will ALWAYS be in the places you currently don't understand. until the day you die, and afterward - this will remain true)

after a while (high negative feedback amplifiers) it becomes aurally tiring and painful, even. It takes time and is also tied to human intelligence and cognition speed, over time. hearing is the same, in that way.

some get it, some never will. Heresy’s don’t really deliver a deep wide sound stage and never will. You were more listening to false edgy cues.

what the tube amp did, to some degree, is illustrate that, clearly. the trick is working one’s way through the inordinately complex logic and data and understanding about ears and gear, in order to ’see it’.

All tubes do is add harmonic distortion (the so called warm sound), so I'm not surprised it doesn't sound as good.

All tubes do is add harmonic distortion (the so called warm sound), so I'm not surprised it doesn't sound as good.

@pedroeb  , I've never heard that tube amps have a reputation for shrinking the sound stage.

I think it’s funny how many assume tubes are somehow superior. There’s a reason SS replaced tube technology for all but the most antediluvian.

That's an odd one. It may be due to your differing amplifier technologies playing the speakers slightly different- enough to not mesh together?

You guys think my ideas were too obvious but ... I have gone through exactly these issues at two different dealers.  The excessively wide Soundstage was due to a speaker polarity flip.

@erik_squires 

right- but did you like it better that way?

@designsfx  - I did not! but it was curious that the OP mentioned how wide his soundstage was before.  Sounds to me like he might have been listening with a mismatched polarity

@erik_squires  , now I understand what you were saying. Out of phase, right? But with mismatched polarity, I would think that there should have been no concrete imaging between his speakers? 

@immatthewj different people hear it differently.  :)  I just wanted to know if the OP had tried that.

I watched a pair of Wilsons get sold to a Russian "businessman" for their exceptional imaging this way.

Gotcha, @erik_squires  , I have never purposely or accidentally done that.  The only experience I have is with one of those test CDs by Rodger Skoff & Doug Sax where (I think it's) Rodger who says, "IN PHASE, my voice should be tightly focused betwen your two speakers," (and it is), and then he says, "OUT OF PHASE, my voice should have no apparent [something or other] and it should sound as if it's coming from you from all around the room." (and it does)  ". . . Move your speakers a little bit at a time to enhance this effect. . . ."

@immatthewj The OP's description of his imaging when it was "good" just sounded too much like that situation.

TBH, I had this happen a couple of times at two different dealers, so I got more than a little suspicious they did so deliberately as a gimmick. 

@erik_squires  it would be interesting to see what would happen if OP had access to and played that track from that test CD. 

The problem should be in the AV preamp.

You can try to set the listening mode of McIntosh MX123 to multi-channel stereo.

@immatthewj  or.... he could just flip the leads on either speaker....

He's hooked something up incorrectly. Tubes ALWAYS give you a bigger , wider soundstage than SS.

If you haven't already do turn off all audio processing on the MX123 including Dolby: Dolby Atmos, Dolby Surround Upmixer DTS: DTS-X, Neural:X Surround Upmixer, DTS Virtual:X Auro: 3D, Auro-Matic, 9.1, 10.1 IMAX Enhanced.  Also turn off Audyssey or any other EQ.  

You guys think my ideas were too obvious but ... I have gone through exactly these issues at two different dealers. The excessively wide Soundstage was due to a speaker polarity flip.

@erik_squires , this got me thinking about an amp I auditioned around 25 years ago. Do you remember the Stereophile that featured The Mesa Baron on the cover? Two monoblocks in a single chassis with all sorts of neat looking things--rack handles, meters, switches, 12 output tubes. . . . Anyway, I had it for a weekend and it was to be an upgrade from a Cary entry level stereo amp. What I first noticed, and what first appealed to me, was when switched to 1/3 triode 2/3 pentode, the soundstage was up front/in your face/ and maybe all over the place. Vocals had a musky quality that I was not used to, & initially appealing to me. But, after I had just about made up my mind to trade in my Cary, I did an A/B and the cleanliness (even though it was small) of the Cary’s soundstage made me feel this would not be a total upgrade.

Anyway, I wonder if there was some polarity trickery going on with the electronic design of the Mesa Baron?

The great designers of our time know how to engineer an amp to get specific but desirable results. I'm still shocked how one amp could do one thing and then just going to a different model of the same manufacturer can produce different results, but it does happen. I vote to put things back the way they were.

Something is amiss. That amp should be able to drive a truck! I’d want to drive it with a preamp. 

Soundstage is overrated. I attend many live music performances. Solo acoustic and electric, medi and large ensembles, a variety of acoustic spaces mostly I hear the sound a a giant quilt devoid of any pin point location. Soundstaging in sound reproduction is an interesting presentation controlled by the production team. I enjoy it but it is a peculiar concept in the realm of live music IMHO