I love my X100-B. Be careful, however. I bought mine via an online transaction and, not being a tech or a professional, did not become aware until later that the amp I purchased had been through a whole lot more than the seller told me. Wasn't till I retubed and had a tech look at it for rebiasing that I found out the extent of the problems. The amp hobbles along but only because I don't gig with it and don't push it at loud volumes. I'm told that to get it fixed it's going to cost (in labor alone) probably half what I paid originally.
As for high gain, everything I've tried with it and all I've researched about it indicate that for 1990's style metal ("scooped mids" etc) it's not going to make it. I regularly use the DS-1 (Keeley Mod) and get more gain that way. I love the AC/DC sort of tone I can get, Frank Zappa sounds, and of course it's great for a Vai tone too. I love the reverb on it. The five-band eq makes it tricky sometimes to dial in the tone I want. Occasionally it's frustrating (especially now that I'm having aforementioned troubles with it) and I still need to get a proper channel-switching footswitch (FS-36 hard to find).
Carvin.com has a BBoard with lots of discussion about this amp and its strengths and weaknesses.