Prepro into a Preamp into an Amp??

yep, yep - simply loop the L/R channel pre-out's from your multi-channel av pre/pro into some auxilary, "tape-in" or "direct"/"bypass" input on your 2 channel pre, which is connected to your 2 chnannel amp. Then, connect all 2 channel sources to your 2 channel pre, like normal, and all DVD,cable,vcr,satalite "video" sources into your AV pre! Then connect everything else like you normally would (Center, sides, rears, and sub outputs from your AV pre/pro are connected to some other amps or mult-channel amp).
Then you will always have your 2 channel pre and amp combo switched on for both music and movies, and only have the multi-channel av pre/pro switched on for movie sources - while 2 channel pre is set to the input your looping in from your AV pre. (also set volume/gain on 2 channel pre at half way, unless you have a "bypass or direct in")
This is the standard 2 channel/multi-channel, audiophile combo system setup, btw - lol. It works just fine.

I run a non HT bypassable tube pre with my HT Pro and as others said its simple even without bypass, just reference volume at 12oclock or whatever you wish then calibrate your HT settings as usual.

What about getting a really good PrePro instead? I was in exactly the same perdicament recently and went with a Krell HTS 7.1. The HTS 7.1 really shines as a balanced pre. For HT it sounds great, but is missing HDMI (not an issue for me since I use a video processor anyway... and yes, I know I'm loosing out on the newer audio formats)

Take a look at the Parasound Halo P7. It has features which make it unique for HT bypass use.

Kal