I was gonna say, short of bringing your completed subs and a store that would allow you to "audition"....For me, I'm looking whether the amplifier has enough output to push these drivers sufficiently. I'm also looking for how it handles delivery of power over difficult passages without needing to activate cooling fans. Any turn on/off thumps in my system. How much current is drawn and if it has an negative impact on anything on the same circuit and vice versa. Practical stuff really.
Given the frequencies I'm using it for now, sound quality should be adequate. But I'd like to also be happy with its transparency if I decide to use it in a different application (e.g. powering large front mains).
Crown XLS fans I've not ever been able to hear without putting my ear literally up to the unit. They're very quiet, never heard them go into a louder mode but if I'm listening to something at high volume, doubt it would be noticeable if it happens at all (it may be a two speed fan setup, don't know as it always seems the same level when I've bothered to put my ear up there). I've not heard of anyone mentioning the fans as being a noise issue. Mine are in the same open stand as my other gear in my living room for the subs, in the small bedroom setup as 2ch my disc player is noisier and its not bad.
I have used them for mains, did that for quite a while before I built my subs. The background noise related complaints I know of were from a few with very high sensitivity speakers (like 110dB), in a quiet room, of some background hiss. I've not had a problem but I don't have high sensitivity speakers (87 dB for me at the time for mains). I've also got one setup (I have four of these amps total) with some 90dB speakers in a 2ch setup, no hiss issues. No turn/on/off thumps.