I have just returned my sony dvp-ns725 and purchased the samsung hd-931 and have noticed a huge increase in sound quality. I was playing the cd's with the sony at -20 for a nice sound I am now playing them at -30 with the samsung unit. The picture and sound quality is also much better on DVDs, even on S-video connections that I am currently using till I can buy a HDTV. Any thoughts as to why such a huge difference? I had always thought that a digital signal is digital signal.

A digital signal is a digital signal yes, but there is more to it than that. How that digital signal is converted to analog (DAC) will be a big difference. There a numerous other reasons, that some others around here could explain it far better than me, but as you've found out, no, not all DVD players are the same.

The main reason for the increased video quality is that the samsung 931 uses a faroudja de-interlacing chip.

Another benefit of the 931 is that it will upconvert dvd to HD resolutions ( on DVI only) for display at your tv's native resolution for the best possible picture. This is most noticable on fixed resolution tvs such as plasma, lcd, dlp.

Question?? Does the 931 actually upgrade the DVD to HD standards. I have read mixed reviews but don't know anyone that actually owns one. The Elite 59-avi is supposed to do the same thing, but it costs about $800 more (is it worth it?).
Anyone have an oppinion on this?

the 931 is capable of 1080i. with HDTV specs changing all the time, the 1080 is as high as you can get right now sent through a DVI connection. And I noticed a much cleaner picture and sound quality with just S-video hooked up on an 8yr old Sony 32". So to me it is worth the extra $.