I too made the switch from a THX (Pioneer Elite ) receiver to a 990 and five 2200 mono blocks versus the 7125. The sound has improved signficantly with the seperates over the receiver. I believe the 990 is a terrific processor and results in wonderful sound. My favorite demo DVD so far is the pod race in Star Wars I. Things were flying in from all directions! I never heard those sounds from my receiver. SACDs (John Mayer, Heavier Things) sound great too!
I currently have M&Ks, but one step below the 150s you are interested in. My 851s, S125C, 55 surrounds and MX-70B are more than enough for my room. I imagine that the 150 system would sound just that much better. M&K has terrific support (sound familiar?) and a call to Barry Ober, their principal engineer, might help you make a choice of their speakers based upon your particular room. He was very helpful when I was determining speaker location (wiring within remodeled walls) for my original K series speakers. About 2 years later he provided great support when I upgraded (via M&K's refurbished store and e-bay) my speakers to the current set.
Good luck with your new Outlaws. Enjoy!
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AvFan
Outlaw 976 | ATI 2005 | M&K 850s Left, Center & Right, Surround-55s, MX-70B Sub | Harmony 950 remote | Panamax 5100 | OPPO UDP-203 | LG OLED55B7