100W+ solid-state amps known for beautiful mids

I suppose one could consider the speakers and cable as a part of a "sub-system" that would include the amp. Depending on that system, the excellent mid range performance you are seeking might take some experimentation. McCormack/SMc can help guide you. I would suggest the DNA series which can be tailored and upgraded as your needs change. McCormack have refined a fundamentally and technically sound design over a long period of time. I have not heard every SS amp, but I do not recall
any over the past 30 years that sounded "better."
You could go with a tube amp, but that can be costly over a period of time with necessary tube replacement. Good luck, happy listening!

@hedonism  What you are looking for is an amp with the right distortion signature. The warmth of the Pass is caused by a 2nd harmonic for example- similar to a lot of tube amps. If you talk to Nelson you'll find he's very aware of how the distortion signature affects the sound of an amp.

If you want solid state you're up against a real problem. Here's the engineering difficulty:
When you apply feedback to an amplifier circuit it will make distortion of its own while suppressing distortion in the amp. The added distortion is audible as harshness as brightness since its all higher ordered harmonics which the ear interprets in that way since it converts all forms of distortion into a tonality of some sort.


Now tube amps actually have more higher ordered harmonic distortion, but it is masked because tube amps also make a significant 2nd and 3rd harmonic which masks the presence of the higher orders. Getting a solid state amp to do this is a bit of a trick with traditional amplifier design.

However it turns out that class D offers a means around this- it turns out that if you used the right encoding scheme, that and the dead time used in the class D amp tend to result in lower ordered harmonics. A lot depends on the design! But if its done right, the result will be an amp with a distortion signature that masks the higher ordered harmonics, allowing for a very smooth midrange and highs with overall lower distortion than you can get out of a tube amp. IMO IME you're better off looking for such an amp- one example is the AGD Audion, which does make over 100 watts into 8 ohms so it should be enough power.

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.

perfection, where did that go ?

but hey, you are happy

all that matters !

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.

High end audio isn't about price, its about intention. Since that amp is ticking the boxes for you, if I were you I'd be looking into (if its older than 25 years) making sure that the filter capacitors in the power supply are good so that the amp remains reliable.