Sherbourn 5/1500A (not the most recent model). Not prohibitively expensive - has balanced inputs - has plenty of sack to push what the 990 feeds it. It's a monoblock design so it's like 5 separate amps in the same chassis.

I got it with my B&K Ref31 but so far it sounds even better with the 990.

Fantastic match - I'd highly recommend it. If you can find an older 5/1500A that works. The latest model is the 5/5210A which you can actually add the extra two channels later to make it a full 7 channel amp.