I compared these two amps on several Proacs, and also the CJ 12's. The comments above are exactly right. The Proacs definitely benefit from the additional power of the 11a, and the 11a sounds more detailed, correct and refined overall. But that EL34 midrange magic of the '55 is not as present in the 6550'd 11a. Also the lack of power of the '55 results in a looser bass, which is definitely less accurate, but can be pleasing if your system is bass-shy.
