"warm" solid state integrated amplifier

Tubeless, many here are sympathetic to your goals. At the risk of throwing in a variable that was not requested, I'd suggest at least trying different speakers (say, Spendor Classic line (not D line), or Harbeth C7). I tried the ProAc D2s some time ago and found that they had a much brighter sound than the older Response models. (I recall a frequency response plot -- maybe in Stereophile -- that showed a "scooped" shape, with a rising treble.)

Otherwise, as others have suggested: Sugden definitely, Luxman class A integrateds, maybe Ayre.

We are assuming that your room is sympathetic (windows covered, rugs, etc) and you have experimented with speaker placement and toe-in angle, of course.

I recently purchased a 3 year old BAT VK655SE in your price range. My most recent amp was a Bryston 4BST and I'm driving Magnepan 3.7i.  The BAT definitely has endless power reserves, slightly better control of the lower bass and fit's your "warm" request.  Never any etching or brightness. The Bryston is quicker with more punch.  The BAT's imaging exists, but it is not locked or precise like the Bryston. I really apreciate imaging, but if that's not that important to you I would Highly recommend the BAT VK655SE.  I am still switch between the two amps and am undecided and as always have my eyes out for my next amp.

CODA CSIB another great reco. Very happy with mine, making me want to move up the line towards the separates. Have had pass and FW in past. 

Definitely less warm / rounded then pass but just a great for tone color and density 

Hi tubelss,

Short answer: get Class A amp or full body midrange speakers like Harbeth

Long answer:  Just sharing my exact same experience at this very moment, the problem you're looking to solve is 'synergy' not just an amp.  As I aged from late thirties to early forties these past few years, I've come to exact same problem that I get 'listening fatigue' much sooner with my B&W 805D3.  I used to be able to listen for 1hr now 15mins start to feel long.  'Bright sounding' is just one aspect, the problem is much broader that just that.  To avoid writing a long paper, below is my bullet points on my journey of progressively honing in on solving system synergy.

1)  Speaker cables, its the biggest impact assuming your system is already at certain above average level.  Audioquest Rocket88 subdues brightness very well but you'd lose details.  Ping me if you want my set.  I'm currently using a 6 AWG cable handmade by someone in USA from ebay that's shockingly good for $400.

2) Finding the right amp.  Big difference between amps that make 100W at 8ohm & 4om vs 100W @8ohm / 150W @4ohm vs 100W @8ohm / 200W @4ohm...etc.  After trying NAD Master, Mcintosh MA5200 a few other small brand stuff, I've settled on Marantz PM-10.

3) DAC, I've recently upgraded to Gustard X26 Pro

4) Interconnects

5) USB cable

Amp alone wont solve your problem.  Good luck with your search for syngery journey.  Just make sure the speakers are really ones that you want to stay with before you splurge 10 grand.

If you can find one used the Primare i30 is a great match with Proac's and should help you achieve the sound you are looking for. I would steer clear of the newer Primare amps as they have gone to class D.

At a much higher cost the Leben CS-600 integrated amp is the best amp I've heard with the D2's.

You might want to touch base with JohnnyR at the Audio Connection. He is a Proac dealer and likes the D2's very much. Good Luck!