I'm not sure if a few bass traps will help. You might want to talk with GIK or one of the other suppliers and get their recommendations. I do have some GIK bass traps but really haven't measured with/without to see what, if any, improvement they provided.

Instead of another set of speakers for HT, maybe some sort of speaker selector if you do get a separate integrated for HT. Or, thinking out loud, a separate 2 channel pre (unamplified) and maybe run it through the Analog ins (with no processing, e.g. pure analog) of your MRX?