Talking about solid state, I've recently gotten my TH900 paired (or perhaps I should say in a foursome!) with an iFi Dac2, iTube buffer and iCan SE amp.
I've auditioned the TH900 previously with a number of other DAC/amp combos (including the HPA8 and Schiit Biforst/Lyr) and while I never had the opportunity to do any A/B testing, my overall impression is that this iFi combo is as good as anything I've heard for under $1500. Very clean, neutral, great stereo imaging.
What I like about this set-up is that it has a few nifty tricks that can compensate for some of the TH900's weak points. For example, while as a rule of thumb I tend to avoid any audio processing jiggery pokery, I found the 3D holographic sound on the iCan SE worked really well on some tracks, giving the TH900 a bit more of the space/soundstage that some might find a bit lacking in these closed-back cans (without overdoing the sound effect and making the track sound cavernous). Adding the iTube tube buffer between the DAC and amp was also a bit of revelation in terms of tempering some of the brightness that can make the TH900 a bit fatiguing.
So what's not to like about this combo? Not much really. Given the whole rig can be put together for not much more than $1000, I think it punches above its weight. For those that can't afford to drop 5 Gs on a dream rig like an Eddie Current 445 and Chord Hugo, I'd say this iFi trio is well worth looking into.
The catch is that this isn't actually my rig (yet). I borrowed it for a couple of weeks from a generous friend, and while I'm pretty impressed with the Dac2/iTube/iCan SE combo, I wonder if I should look at some other set-ups before making a final decision. I've yet to test any Woo Audio gear, which I know some speak highly of. The new iFi iCan Pro (has it actually been released yet?) sounds pretty intriguing too. Would be interested to hear from anyone that's tried it out with the TH900.
As for myself, I'll report back if I go with another set-up!
I don't know that this would suit you, and hopefully I'm not hurting any feelings by posting this here, but there's a combination I would consider that has really impressed me personally. I'm not going to recommend a DAC with it as I figure you can work that out on your own (I use an Ygg which is as good as I have come by personally).
I rediscovered my Senn HD650's after I purchased a Head-amp GS-1 with the Dynalo+ upgraded boards. They don't come up for sale often, but if you can buy the amp well, and get your hands on a pair of HD650's (or 600's if you prefer that signature), I'd recommend the pairing. I have a Woo WA7 that's no slouch, but the GS-1 Dynalo transformed the 650's and I'm assuming it's through both transparency and sheer power. I haven't been able to put them down aside from switching to my e-stat rig as I love it as well. With guitar music (blues, jazz, R&B, classic rock) it's one heck of a pairing. If you really need the earth shattering sub-bass, it's hard to beat the TH900's so I don't want you thinking that if it's what you are really after.
Best of luck in your search.