Another vote for the M60. The main advantage of horizontal centers is that they are, well, horizontal, so you can put them on top of a TV. If you can put the center behind the screen then a third M60 is definitely the way to go.

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