I can second the Marantz. I'd also suggest a Pioneer. I have not compared these head to head with each other but I own a digital Panasonic XR-25.
I have heard all of these separately and I think the Marantz is a clear winner. I installed the Marantz with some JBL HT in a box type speakers that are OK for the money but not anything special... I installed the Panasonic with some excellent small surrounds from a custom company in Georgia, www.rutledgeaudiodesign.com. The Pioneer was connected to a pair of B&W CDM NT9's. I didn't get to hear it in surround but, I'd say the Pioneer sounds marginally better than the Panasonic but has a lot more features. And I know it could be setup to outperform the Panasonic overall.
I have heard NAD in the past (1995) and wasn't impressed at the time. I've never heard a HT setup of NAD gear. I've heard a lot about it though, which means nothing in reality. Marantz gear went downhill for a while and now it's a lot better in quality. NAD has probably changed... it's been almost 10 years. It could be better or it could be worse than what I heard... There was a lot of talk about the Panasonic digital stuff but I don't think it sounds as great as it's hyped. I've bought a couple of items based on reviews that really don't deserve the hype. Trust your ears.
Until you hear some of this gear, you won't know how it sounds. See if you can get some demo's from a local dealer or buy used here.... and resell what you don't like.
However, based on what I've actually heard and seen, I'd suggest the Marantz SR-6200 or the Pioneer VSX-912. If someone tells you A is better than B, find out why they think so.