The X250.8 is a class AB amp with a fair amount of bias current. The idle power dissipation is 450w. With higher efficiency speakers, you probably will be in class A bias most of the time, so power dissipation isn't going to be that much higher than the idle dissipation.
However, your Harbeth speakers are not that efficient at 85db/w. I'm surprised the dealer felt that a 25w or 30w amp (rated power) would be the best match.
Both the XA25 and XA30.8 are class A amps with high bias. The XA30.8 idle power dissipation is 400w, almost the same as the X250.8. But as long as you are staying in class A bias, it's power dissipation will not go up while playing music (in fact, it will go down a bit). The XA25 has lower bias current and idles at 240w. The XA30.8 has higher peak power in class AB than the XA25, but I doubt that you will notice that much difference between the two. One other difference is that the XA30.8 supports balanced inputs, while the XA25 does not.
It's impossible to say how much your room temperature will increase with these amps since it depends on the size of the room, insulation, other heat sources, outside temperature, etc. I have a pair of XA60.8 monoblocks, each of which idle at 425w. I haven't owned them during the hottest months of the year, but so far, I haven't noticed that my room is any warmer (without AC being on). But my room is fairly large at 29' x 17' x 8'.
However, your Harbeth speakers are not that efficient at 85db/w. I'm surprised the dealer felt that a 25w or 30w amp (rated power) would be the best match.
Both the XA25 and XA30.8 are class A amps with high bias. The XA30.8 idle power dissipation is 400w, almost the same as the X250.8. But as long as you are staying in class A bias, it's power dissipation will not go up while playing music (in fact, it will go down a bit). The XA25 has lower bias current and idles at 240w. The XA30.8 has higher peak power in class AB than the XA25, but I doubt that you will notice that much difference between the two. One other difference is that the XA30.8 supports balanced inputs, while the XA25 does not.
It's impossible to say how much your room temperature will increase with these amps since it depends on the size of the room, insulation, other heat sources, outside temperature, etc. I have a pair of XA60.8 monoblocks, each of which idle at 425w. I haven't owned them during the hottest months of the year, but so far, I haven't noticed that my room is any warmer (without AC being on). But my room is fairly large at 29' x 17' x 8'.
