Will this amp sound good as a subwoofer amp driving a 96dB 8ohm subwoofer? I mostly listen at low volume, so I'm suspecting that my 250W classB amp isn't ideal. Is the benefits of classA as big in the bass range as in the highs and mids?
I'd go the other way around. Use your 250w Class B amp for the sub and the a40 for your mains. Reproducing lower and lower frequencies at the same volume level consumer greater and greater power out of the amp. I don't remember the exact details, but it goes something like this:
To create a 40Hz tone at a specific SPL requires twice the power as reproducing an 80Hz tone at the same SPL. Using this same rationale, reproducing a 20Hz tone then requires 4x the power of an 80Hz tone.
Again, I'd don't recall the exact relationship, but it goes something like what is above. This relationship dictates that you put the larger amp for your sub and use the A40 for your mains.