Tube Preamp Recommendations?

Thanks for the responses, everybody.  I've spent a ridiculous amount of time researching pre-amps and it had gotten a bit overwhelming for this newbie.  I appreciate all the thoughtful comments.  

Kalali, the impedance of the ProAcs is 8 ohms. The NEW A-20 amp is rated at 20 watts at 8 ohms.  I must confess my ignorance about whether the amp has low input voltage sensitivity and/or the speakers are efficient/sensitive.  Do I need a preamp with some type of gain reduction given that i listen almost exclusively to the Aires Mini?  At some point I'll upgrade that to a higher end streamer plus stand along DAC.  

My budget is about $1,600.  The Allnic L4000 looks great but clearly out of my price range.

Mechans, I am definitely looking for some 3D magic.  I tried an SP9 (I realize it only has one tube in the line stage) since it was easy to borrow, but the sound was too bright and sterile for my taste.  I'd hoped an SP6 or SP8 would be warmer.  Sounds like that's an incorrect assumption?

Are the AI Modulus 3A and ModWright LS 100 on the cooler or neutral side too?  It looks like tube rolling on the AI isn't really possible (lots of people report burning out tubes quickly when they use tubes not recommended by the manufacturer), but is possible with the LS 100.

Gdnrbob, the Atmasphere looks interesting, but the exposed tubes won't work in my home.  Is there another 6SN7 preamp that might fit the bill?  6922/6dj8 seems to be very popular.  Are the 6SN7s warmer?

Thanks again!

Thanks, jond.  I'll check those two out.  I'm definitely looking for a used piece to get the biggest bang for the buck.

Thank you, bdp24! I got the AR SP-6b.  Looking forward to listening to it.  Thank you to everyone who responded.  What a great community!

Bdp24, do you like to Telefunkens better than the Phillips Jan 5751 that it comes with?  Also, what is the benefit of the mode switch?  I figured you'd set it to stereo and forget it?

Sorry dryanlocker, I don't have a manual. I just looked at the remote and don't see any writing that would indicate codes.

Thanks for all the responses. I indeed purchased an AR SP6b.  The sound is wonderful.  I also purchased around the same time an AI L3 dealer demo directly from AI.  AI is upgrading the power supply and some internal caps so it will more or less be an L3A.  The unit still hasn't arrived.  I'll post my impressions after I receive the AI.

Thanks for your patience.  The L3 took a lot longer to arrive than expected.  It is an L3 dealer demo that went back to AI where it was essentially upgraded to an L3A and then sent to me.  

I love so much about the AR SP6b, particularly the bloomy midrange that works so well with my ProAc Response 2S speakers.  It especially shines with vocals and acoustic instruments.  It feels like the tubes are backed by an enormous reservoir of power that effortlessly pushes out warm notes that suspend in the air for just a moment before the next note.  I can listen to the thing all day long; it's really enjoyable.  What it is missing, however, is detail, particularly in the high end, and imaging.

The L3+ has that detail that the SP6b is missing, along with the imaging. The build quality is exceptional.  But it feels dry, flat, or maybe sterile, compared to the SP6b.  It's probably more accurate than the SP6b.  In the right system, which would probably be mega $$$, I might prefer that accuracy.  But in my system, it just doesn't move me.

So, how do i get the midrange bloomy sound of the SP6b with more detail and better imaging?  Folks on this thread mentioned above:

1. Mapletree Audio Design Ultra 4c 
2. a Don Sachs preamp--not sure which
3. Atma-sphere MP-3

Would one of those would fit the bill?  My budget is $2k (notice the creep from my original post.)  

The other alternative is to overhaul the SP6b--I suppose new wire and caps might brighten it up, although i don't know if that means more detail and better imaging.  Also, i'm not sure who could do it, and I'm not sure that i want to do violence to a vintage piece of equipment.

Please bear in mind that i've done some reading, and a little listening, since my original post, but i'm still very much new at this thing.  You're hearing my subjective listening experience.  Apologies in advance if i have maligned a piece of equipment that you hold dear.  Folks have been very generous with their time and advice which i really appreciate; some have been grumpy when i don't respond right away--please understand that life circumstances prevent me from being tethered 24/7.  Thanks so much for any thoughts.

Mechans, thanks for your post.  It's really interesting b/c your take on the SP6B is very consistent with what I've read in other places.  In fact, the drivers in the ProAcs developed torn surrounds a few months ago, so I'm about to send them to Millersound for new surrounds.  Perhaps that is influencing things; the dried out surrounds certainly killed the bass, but I didn't think that would affect imaging and the high end.  I'm going to hold off on further preamp tweaks until I get the speakers fixed.  

Joey54, the PSE is gone--a friend took it.

Giri, yes, i'm very interested in a 6SN7 preamp.  I'm very interested in a Cary or perhaps a Modwright that uses that tube. Thanks for the tip.

Thanks, hifiman5.  That went fast.  I'm leaning towards a 6SN7 preamp, but maybe I'll check out a CJ.  Detail and "air" has improved with an AC line conditioner.  Oh, and correcting the inverted speaker polarity once I disconnected the AI preamp. :)  The AR SP6B now sounds quite a bit closer to how mechans described.

@croatan128, your experience mirrors mine. A few months ago I put in new 1.0uF Vcap ODAM output caps and it took the pre to another level, which I didn’t think was possible. More detail and more intimacy—like I’m sharing space with the vocals and stringed instruments I’m hearing right in my living room. I coupled the pre with Don’s KT-88 amp and the pair are just divine. Other parts of my system have since come and gone, but I have no intention of parting with Don’s pre and amp. Enjoy!