I tried using the HD500 just as a stomp box (no hd500 amp) in front of the Peavey C30 today...it sound terrible. There's just no body to the stomp boxes at all, very thin and fizzy sounding....guess the honeymoon is over, or maybe the marriage :-(
The stompboxes (or at least the OD/Distortion ones) are the weakest part of the HD's. The amps are the strong part. Several guys have commented that they are using their regular pedal boards in front of them or at least a few ODs. The mod pedals aren't bad. There is a lot of talk that new "HD Pedal Models" will be one of the first model packs released. Still, I'm not dumping it because of the pedals. I bought it for mainly the amps. IF I really had too I'd put real pedals in front of it. I can get most of what I want without resorting to that too. I have not used it "pedals only" in front of an amp though. Got a nice pedalboard full of boutique pedals sitting right next too it, so haven't even tried it.
Actually, I'm a bit premature there, the marriage is back on. Howie, you are correct in that the amp sims are good and the stomp boxes are not so good. The stomp boxes do a reasonable job with the hd500 amps, but trying to use just the stomp boxes in front of a real amp just doesn't work, everything is voiced so thin with no body to it.
The HD500 is still very good for recording though, and I will try it with some real stomps this week and see how that fares.