The 990 is an overall neutral piece of electronics, so you can basically just focus on the most important factor in any speaker shopping exercise: what sounds best to you. Get some CD's you're really familiar with and take them with you to demo any speakers you are considering.

Another thing to consider (although it's less important than personal taste) is the amp being used - any separate power amp should be easily able to drive 4 ohm loads, and if you have a fairly powerful amp you aren't likely to be particularly worried about speaker efficiency, but if you are planning to use a smaller amp (something comparable to Outlaw's 7075, which puts out 75W per channel) you might want to avoid something that's got a particularly low efficiency.