Hello, and welcome to the forum.
I have the T973, and traded in my T163 for some new HT mains. Nice stuff, ain't it? I am not sure that the benefits will outweigh any degradation. I am not an expert on this. However, the NAD should have a superior analog section, which definitely counts for something. The new audio formats will give you greater dynamic range. I think you're covered on the amplifier part of the equation, so the rest is essentially up to the speakers, sub, room, and setup in this regard.
I bought a refurbed 805 from a President's Day sale at Onkyo Shop for $600. If I was buying a unit today, I would very strongly consider Yamaha. At the time of my purchase, there were not enough details available on the Yamaha line, and I felt like I had been waiting long enough for hi-def audio support. (My bdp is a Pana Bd-30, and so bitstream is my only option).
The Onkyo's, at least the 805, run very hot, unfortunately, even just as a pre-pro. I have it in an open audio rack, but it does make my NAD amp feel like an ice block in comparison. If you are using yours in some sort of cabinet, or credenza, I might look elsewhere. Ok, I know the 705 will not run as hot, but I think the overall QC issues with Onkyo are something to be concerned with. I tried calling them before, and it appears that CS is very, very lacking. Why did I buy the refurbed 805? Well, there weren't really any other afforable alternatives at the time of purchase, and the 805 was priced very similarly to the new 705. I figured wtheck.
I was also able to make this leap in compromise, er faith, as I was able to split off my electrostats into their own 2-ch room. I also needed an HDMi switched of some sort, and I figured this cost was defrayed in the receiver purchase as well. There are those who believe there can be a syncing issue with Onkyo's, but I myself am not sure if this is true. Could be though. Any sync or dropout seems to be more likely when one is outputting 24 fps video and bitstreamed hi-def. The only time Ive had issue thus far is with Sunshine on BD. However, many people are having the same issue with this disc when using 24 fps + DTS-MA bitstreamed, whether using Onkyo or not, and that's why I can't say its the receiver for sure.
I know it appears Im not answering anything simply for you. I think outside of very particular needs or setup constraints (ie hdmi, switcher, convenience, bitstream), I would make sure that everything else is up to snuff first. Including speakers, sub, treatments, speaker positioning, etc.
If you use MCA output from the BD-50, find out what kind of cut, if any, there is in the LFE. Bass mgmt gets above my head at times. For instance, how is one supposed to increase LFE back to "normal" without simultaneously increasing any summed bass? I'm sure there's a way, I just don't know it.
Something else to consider is the afforable HT pre/receiver coupled with a 2-ch pre/pro with HT bypass. You can add that later down the road. You will want unity gain with the ability to bypass while off, so that its not imbuing its own sonic signature during HT playback. If you are a big multi-ch music guy, which I am not, I would just ignore all of the above and either keep the NAD, or when you upgrade, get the nicest you can afford.
If you are curious on the 663, I think I know how you can get it for $400 new. At least as of a few days ago, they are probably sold out now, but you very well may obtain this price on backorder from an established vendor. PM if interested.
Other tidbits. Both the 663 and 705 can matrix hi-def over 7.1. While the 705 can have differing x-over points per speaker, the Yammy cannot. The improvements from the 705 to the 805 include use of BurrBrown DACs, current of 60 amps vs 36 amps, THX Ultra 2, Zone 3. Otherwise, they are pretty darn similar.
-J
