Opps, stupid me, didn't know it was a seperate product. <g>
While there is no evidence of hd(or 5.1) support (or lack thereof) on their site, judging by the contents (nowhere is HD or 5.1 mentioned that I could find for instance), and the wording of stereo seperation, I got a feeling, if it supports the audio connection in use in your system it will do it's job, now as far as I can tell, the regulator is meant for stereo, so it may pass the 5.1 along, but probably won't normalize it (except maybe 2 channels), or it will drop the 5.1 completely and mash it into 2 channels, if it passes the 5.1 and only affects 2 of the 5 and normilzes the 2, youll have 2 of the 4 in 5.1 normalized, the rest not normalized, if that's the case, then you might be able to run a few of these in parallel, depending on your wiring to normalize all 5.1.
In short, it sounds like it won't matter if it's HD or not, but, it will matter how the audio is delivered to the regulator then to the audio system, and it may or may not pass 5.1, if it does it will only regulate 2 channels, or I could be way off, just giving my input. Probably would be best to ask the makers of the product.