Aside from the heat consideration, you must keep in mind that the XA25 and X250.8 are different in their design (single ended vs. balanced) and will have considerable differences in their sound. I seriously doubt that the 250.8 would be "too powerful" for your Harbeths at 85 dB sensitivity. Regarding the sonic differences, I have been told by the folks at Pass that the 250.8 is more extended on the top and bottom end but doesn't quite achieve the rich midrange that the XA25 delivers.
Heat from amps?
Looking through the forum threads on amplifiers I’ve read posts stating how the Pass Labs x250.8 gets warm enough for the occupants in the room to take notice, along with comments of being thankful for having an air conditioned room. In degrees, how much are we talking about at what levels of volume? I’d like to take all things into consideration while comparing power amplifiers. I have not been playing my Bryston 4B3 demo at loud volumes for long periods of time, but, as of yet, and the heat sinks have only gotten warm to the touch.
update : Keith English at Pass Labs had me call the dealer who I,ports Harbeth. "The 250.8 is too powerful, the XA25.8 is what we recommend." When asked about the XA30.8 I was told there is little difference compared to the XA25.8," and they reiterated the recommendation of the XA25.8 which has more than capable for handling my Harbeth 30.2 XDs. I thought dealers like to up sell? I'm worried about the handling of the bass on the XA25.8.
update : Keith English at Pass Labs had me call the dealer who I,ports Harbeth. "The 250.8 is too powerful, the XA25.8 is what we recommend." When asked about the XA30.8 I was told there is little difference compared to the XA25.8," and they reiterated the recommendation of the XA25.8 which has more than capable for handling my Harbeth 30.2 XDs. I thought dealers like to up sell? I'm worried about the handling of the bass on the XA25.8.