Does the UBR62 or any other speaker you shortlisted known to have trouble playing mid tones while also reproducing bass at the same time? If so, it may not be a very good speaker to begin with. But then again, as I see it, the UBR62 is a three way design so each driver has its own frequency band separated so I don't understand the requirement to LPF.
Secondly, I don't see any reason why an amp shouldn't be using as much power as it is rated to make unless it has a flaw with its design which cooks it.
But since you don't want to use all of the power, look at the Bluesound Powernode which has only 50w x 2 but then again, all the power will not be required as you'll be using an LPF while coupling it with a Sub.