100W+ solid-state amps known for beautiful mids

Thanks for your wonderful suggestions guys, keep 'em coming! 

To maybe help me narrow my search, I'd like to add that a tube preamp is a must in my personal system, and since the majority of tube preamps are single-ended and output-capacitor coupled, I'd like to specify that ideally, the power amp should have high input impedance and have RCA inputs!

For reference, my current system consists of:
Rockna Wavelight R2R DAC
Vacuum Tube Audio SP14 6SN7 linestage
PASS labs X250.5 power Amp
ATC SCM19 v2 speakers

@blackbag20 A difference of philosophies, I reckon. I'm of the opinion that everything is coloured. "Wire with gain" doesn't exist in my books. To me, it's all about mixing and matching components to offset weaknesses and create synergistic sound. 

Does anyone have experience using Ayre single-ended? I realize that best results are achieved using a fully balanced set-up when it comes to Ayre gear, but my tube preamp is single ended, and it’s going nowhere. I think my Rockna DAC is internally single-ended as well.

@sumaato thanks for the tip on the quad 405-2, I will buy one to keep as a back-up amp, and possibly even my main amp if it's much better than my old Quad 909!

@avanti1960 Nope! While this post is to help me decide which direction I want to move for my next amp, it's always great to learn more about the all-time great amps of yesterday. Buying used is also always an option.

@mikelavigne I deliberately didn't include a price range, because I just want to learn more, not necessarily decide which amp next right away,

@fstein 

What is wrong with the Pass you now have?

It's an excellent amplifier and I've really enjoyed my time with it so far, but it's not flawless.

1. It's a very high performer, it's got a very spacious soundstage, wide deep and tall, with excellent layering and separation. It has large vivid and palpable imaging. The bass is huge and lots of fun. But at the end of the day, it's slightly too coloured for me, and despite the warmth, the mids still lack some of that sweetness and purity that the Quad had. There's still some dryness to the PASS X250.5's mids.

2. It has quite low input impedance at 12k ohms. That means there's some bass roll off with the tube preamp that I have currently. I could ameliorate that with a higher uF value output capacitor, but I would like to avoid soldering and I'm worried what a higher value cap would do to the sound. Buying a new amp is just easier, one with higher input impedance this time.

3. It takes quite a while for it to warm up and reach peak performance. I don't like waiting around that long, especially when I've had a long day at work and just want to kick back and listen to some music. I'd also rather not leave it on 24/7 because it's an older unit and I don't want the capacitors to dry out (plus the energy bills add up over time!)

@gryphongryph I am using VTA SP14 which is the predecessor that Don Sach's is based on. My speakers are ATC SCM19. The founder of ATC, Billy Woodman recommends 100W+ with high damping factor on his speakers, so I'd like to adhere to his recommendations. For the reason, I think it's headroom, since transients require a lot of current.

@ghdprentice I believe that my speakers are incompatible with the vast majority of tube amps. I'm taking advice on amplifiers based on the ATC founder Billy Woodman, in that ATC speakers are best with 100w+ class a or ab solid state amplifiers with high damping factor. I think the problem is that most tube amps have extremely low damping factor, which apparently doesn't mate well with ATC speakers, though I have yet to try myself. So for now, I'd like to stick with a solid state power amp with a tube pre-amplifier to get at least some of the tube magic in! If that doesn't pan out, maybe I'll try one of those monster 100W+ tube amplifiers like the Rogue 100, and if that doesn't work, then I might move on from ATC and try something more tube friendly like ProAc bookshelves.

Thank you all again for your excellent suggestions. I'll keep them all in mind.

Here's what I've done in the meantime: I've picked up a vintage Quad 405-2 to use in the interim. And it is fantastic. It's got the sweetness in the mids, much like my old Quad 909 did.

The Pass X250.5 surpasses the Quad in quite a few areas; namely it has more of a wraparound enveloping stage, more authoritative bass, better separation and layering, larger image size. In general, just a bigger and bolder sound.

But the Quad definitely trades blows! It's more tonally neutral and doesn't homogenize the sound as much as the Pass. I've found that the nuances of the original recording are better preserved with the Quad. The Pass had a tendency to transform all recordings into a larger-than-life version, which is awesome at times but perhaps unfaithful. The Quad also has a more refined smooth treble than the Pass in my opinion. Most importantly, the Quad has a better sense of immediacy or urgency, whereas the Pass could almost sound too relaxed at times. 

The best part though? The Quad 405-2 cost me $400. I've generally had in my mind that price correlated with quality (though not ironclad), but this recent pick up is seriously challenging my beliefs.

This pick-up will allow me to sell the Pass with no regrets, and further to that, I feel no urgency to pick up another amp. I'll really take my time to consider my options and wait for the right opportunity to find a replacement amp in the upper echelons.