I've had 10 originals, and 1 2555x. The X is very close to the originals. Only difference is at high vol, where the OG does sound a bit better. The RI can also take any quad, or 2 different pairs of octal power tubes. 2 EL34s with 2 KT 88, 2 6L6 with 2 6550s, etc etc. Like Jeff said, they have more of an emphasis on the lower mids BUT can still bring the upper mid KERRANG when dialed brighter. The EQ is VERY responsive, almost like an active eq vs a typical Marshall eq response. A very good clean, as well with a nice loop.
Great amps...boosted, one particular 2554 combo I had was as good as a Cameron 2203 I had at the time.
