Okay, so the best amp for these I've heard is with my Zodiac Platinum, but that's rather impractical.
Second place is the Fostex HP-P1. Bass isn't as well controlled and overall it's a warmer signature, but it produces a smooth, velvety sound that I could listen to for hours and hours. I'm thinking the Fostex TH line is tuned using this DAC/amp as a reference, at least in part, because it's just such a complementary pairing. Yeah, I'm aware of the low damping factor, but sometimes the whole is greater than the sum of its parts (aka. "synergy").
Third place (tie) is a surprise, the FiiO E18. Seriously, it does an amazing job of driving these and is the value winner, hands down. Overall it's neutral, neither too cold or warm, and even though bass isn't nearly as tight as with the Zodiac, and it's not as detailed overall (no surprises there), it's still a very musical, pleasing experience, for 1/35th the price. Right in with the E18 is the Pico Slim, which to my ears is nearly the same. It's slightly more revealing with a touch more bass control but it's very hard to tell unless you're doing A>B switching on specific passages. As far as value, I don't believe the small differences are worth choosing the Pico Slim over the E18, at least not with the TH-X00s.
The O2? It's cold, harsh in the upper treble, and even though it does a good job of diaphragm control, the bass is lacking in texture and quality. I'd avoid that pairing.
PS. I don't have my Grace m9XX yet, might be arriving tomorrow, and the above ranking could change, we'll see.