I'm thinking of a Yamaha RX-V750 or Denon AVR2105 to match the Epic 50 home theater. I wonder how much receiver do I need to get good sound? Would a lower end Denon (eg AVR1505) or Yamaha RX-V550 be sufficient or am I gonna kick myself in less than a year?

I can't comment on the two receivers you have questioned, but I'm quite happy with the way my Onkyo 602 mates with my M50s.

Any of those receivers mentioned will get the job done. Just make sure you get a receiver that has the features you need/want.

I also suggest you add pioneer and harman kardon to your list. Find the best price/performance ratio between these brands and then you'll be ready to make those m50's sing.