So if I understand correctly your HD-PVRs are only able to record analong stereo audio? I would hope that is something that would be fixed soon, I wouldn't be happy with that at all. I am able to switch back and forth between 5.1 SPDIF and stereo analog for my recordings.
I suggest you give Windows 7 a try; it is quite a big step up from Vista even though I never tried the TV pack. Win7 has native support for MPEG4 as the HD-PVR records in that format but is then wrapped in the MS .WTV wrapper. Win7 is also able to natively playback H.264 .MKV files as long as an appropriate splitter is installed.
I like Win7 so much I have it installed on my office PC, my Acer Aspire One netbook and my HTPC. The Homegroup and Libraries features as well as the substantial upgrade with WMP12 make it quite a lot more enjoyable to use compared to Vista. I can see a lot of plugins making use of the DLNA certified WMP12 and Media Center once Win7 is released. Already the PlayTo media streaming and control are great for whole-home audio and it requires no extra programs to install. WMP12 even allows for streaming over the internet as a standard feature as long as you have a Windows Live ID.