New tubes...far more power??

If old power tubes are worn out, new tubes will sound significantly louder. 

My brother was visiting over holidays and noticed high and bass were missing in my system.   Probably I was used to ... so replaced 16 KT150s and SQ was restored.
Also it takes ~24 hours for KT150 to settle / re-break in.  My tube vendor burns tubes ~200 before testing but doesn't good until ~24 hours of use.

Aside from the possibility that the older tubes were substantially worn out, such that the difference in gain was very significant, it is hard to account for why the new tubes have more gain (not necessarily more power), yet sound so much less desirable.  Is it possible that the tubes are not the same in design or build?  The manufacturer could have changed up something between the two batches of tubes.

In some of the amps I've heard that use the KT150, the sound is a touch too brittle and hard sounding for my taste.  Perhaps, as the tubes age a bit, they soften in sound and you might be someone who prefers that softer sound.    

@don_c55 you are right, I am a tube novice, having owned tube amps and preamps for more than thirty years.
Plenty of assumptions here. Including that these tubes need to be broken in..
as they were played for about 200 hours when I first acquired the amps (Jadis mono blocks) and were then put aside in favor of the stock KT150’s that came with the amp.

What is the brand and model of the tube amp?

Yes! Tubes break-in and age during the first 50 hours of use.

Davey f you must be a tube novice IMO.

I will try the unplug suggestion, will be using my back-up amp ( a ss design, which does make for a lot less nervosa) for a week or so.
Last night, I was pleased to hear that the tube amp was in fact sounding a lot better, the increase in power was still very evident ( and strange) but the overall SQ was fuller and less bright. I moved a power cord off the preamp and replaced it with another brand...this could have helped.
Here's the interesting thing, my ss amp puts out 250 watts/ch into 8 ohms...and nearly doubles into 4ohms, my tube mono blocks put out 48watts/ch into 8 ohms and not much more into 4ohms...yet the tube amps sound FAR more powerful than the ss amp. ( Yes, the tube amps have massive transformers, while the ss amp has slightly smaller transformers..and the tube amps are mono blocks, while the ss amp is a stereo amp...but the power output in volume is strikingly different...and adverse to what one would expect!) 

Have you unplugged the amp since removing the old tubes? Your auto-bias circuit may be retaining the settings it used with your old tubes. Sometimes unplugging the amp resets to "factory settings". Leave it unplugged for an hour or so. I'd even remove the new tubes after unplugging ( plug them back in before restart). 

I am not aware of anyone manufacturing KT150 tubes other than Tung Sol. As far as tubes not inproving/settling in with some burn in, is not what my ears tell me. Just sayin.

They do have a require a little break in time , just like anything else does.
Probably 50-100 hours

And if you think that your power tubes will sound better with use - forget it! Tubes, unlike say, wine and whiskey, do not improve with age!

All tubes, both large and small, produce varying amounts of harmonic distortion: 2nd, 3rd, 4th, 5th ... And this is measurable. Your new set probably has a different harmonic distortion signature. Which you noticed and did not like! There is no "cure" for this! Just accept it - or buy a set from a different manufacturer. And hope that they will have more pleasing sound! 

This amp design doesn't allow for manual biasing. As I said before, I don't know of any auto basing designs that allow for manual biasing..do you??

If the manual doesn't cover this, I'd contact the maker. Regards

@dweller   My amps are auto biasing, but they have no method to manual bias, I don’t know any auto bias amps that do.

Does your amp have auto-bias? If so, you may have to reset and bias to spec for new tubes. The auto-bias then maintains as needed. Check your manual.

Hard to say. I've replaced a lot of sets of tubes over the years. Gave up on tube rolling after the first few tries proved its just not worth it. So almost always mine are only replaced either after quite some time (years) or they get noisy, or once in a while flare out. The one time I had an experience like yours the tubes had run a very long time, maybe 5 years, and then the new set seemed quite a bit more dynamic and loud. But not strident. Never had that. Not even close. Why we have tubes in the first place, to avoid this. 

Hard to think of any tube that could legitimately be called strident. But that would seem to be our only option. That you got some bad tubes. What did you have, and what did you replace them with? And strident, what exactly do you mean by that? Not quite as sweet as before? Or grating and hard?

+1 Yogi...that was my first thought. If the old set was "worn" then the new set would be noticeably more robust in dynamics.

Are you sure the new tubes are not over biased?