I have a setup close the the Epic 80, minus the '80s. So I can't speak to the mains. As has been well-documented here, the EP350 sub can be a real bear to position correctly. But when you do get it right, it is a fine all-around sub. A little better than your Hsu on the music side, not quite as ballsy on the HT side. The QS8's are flat-out superb surround speakers. They just fill your room with sounds that appear out of nowhere. I had my own problems getting the VP150 positioned correctly, but it produces crystal-clear dialog without the midrange coloring many other centers can produce. A note- check out my EP350 vs. VTF3 Showdown post in the HT forum. I actually thought it was a MKII until someone here asked a few questions about it. I think you would be well-served to keep your own sub and try it out with the new speakers before worrying about upgrades. The EP350 in my opinion would be more of a side-grade. They are pretty close in performance. Although you would miss out on the 5% complete-system HT discount. Another thought: If you had the M80's you might listen to music with the sub turned off. Or at least with the receiver set for "large" fronts so low-freq goes both to the sub and the M80's. Anyway, you would really have to move up to the next class of sub to make it worth your while (and money) in my opinion. No, forget it. Get the EP500 so we can hear more about them. They are still new and we're always wanting to hear more feedback on them.
