If you buy just the 990 and decide the Carver wasn't enough, I think you can still order the 7125 and get the combo pricing - as long as you do it within the 30 day trial period. Just confirm this with the Outlaws before you order the 990.
Of course, you could just order the 7125 and do an A/B comparison with the Carver which is probably the only reliable way of knowing whether spending the extra $ is worth it. Sure, you might be out the shipping cost of the amp if you decide not to keep it, but you have to weigh that against the time you'll spend agonizing over the decision and wondering if you made the right one after it's all over with. How much is your time worth?