Although I don't have an 800U, it is my understanding that its THX setting adjusts the picture to the Holy Grail movie setting of D65. If this is so, then what I have to say here may be of interest.
I bought a Pioneer Kuro PDP-6020FD last month, mostly because the 58 inch 800U wasn't available yet. Through the various Kuro threads here, I found the settings for my 6020 that were very close to the D65 standard. At first, I thought it was a little dark but as I got used to it, I saw how remarkably good it was, particularly when watching darkly lit scenes in movies. Now, I use those same settings for all my HD viewing, even sports in a normally lighted room in the daytime.
The point of all of this is to recommend that you live with the THX setting for a few days and see what you think after that. If your experience turns out to be anything like mine, you will soon prefer it to all others for HD. That said, SD programming does seem to benefit from settings that are a bit more vivid than the movie standard.