For that room, that system, and that mix of movie/HT, I don't think you're going to get a lot of benefit from any upgrade. If you were going to upgrade anything and actually notice a difference going from EP500 to EP600 would probably make the most difference but (a) the difference would be quite small, (b) WAF on an EP600 is probably not high, (c) you can bank WAF for the future by spending the upgrade $$ on something your wife would really like...

If you are still looking for excuses to upgrade, the best line of reasoning I have found is the "We're probably going to move into a larger house in the future" ploy.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8