DAC on Universal player

I needed an outboard dac for other reasons and ended up with the MF Trivista 21. I also wanted to delve into hi rez digital and have the Marantz 8400 (decent redbook playback but not at the same level as my Naim CD5) therefore I connected it as a transport to the trivista 21 and that, to my ear on initial impressions, is better than the Naim CD5 (without flatcap). As stated above the SACD output is only analog, no digital DSD stream, so you are left with the universal player's DAC for SACD. However, the DVD-Audio stream can be sent to the outboard DAC for potentially enhanced digital-analog conversion. Overall, I think the outboard DAC can enhance your listening experience if you have a marginal redbook player(the trivista 21 may even outperform an excellent redbook player such as my Naim CD5). My understanding is that the DAC will have no role on SACD playback and may or may not improve your DVD-Audio playback. Hope that helps.
BTW after an extensive A-B comparision I thought the MF Trivista 21 DAC connected to a transport played back redbook CDs better than the MF Trivista SACD player-but not by much. Hopefully, it will do so connected to a music server.

Is there a used DAC for $400 - $800 that would be an improvement over what the Denon's internal DAC's do?1

I don't think there is a usd DAC in the price range you just mentioned that would improve things enough to be worth it. I agree with the folks who said that modding the player will make the greatest improvement, and not only to redbook.

Sell the denon, buy the marantz universal (dv8400 or SA12v2). That will give you a very appreciable improvement on all formats for very little money. Denon doesn't make music very well at all with any of their players, they can only seem to get video right while leaving the musicality out....That said, if 2ch music is what matters most, having a DVD player in a seperate box is the only way to go if you have less then $6K to spend....

I recently purchased the Tri-Vista 21 and it made a marked improvment over my Denon DVD-3800. I now use the DVD-3800 as a transport most of the time exept for DVD-Audio. I think the biggest difference is in the smoothness of the midrange and treble regions. Also, the bass seems to be more solid and defined. To help you gage this response, I am in the middle of a major upgrade cycle so I still have a lot of tweaking to do to get things just right. Right now my system consists of...

Revel M-20
BAT VK-300x SE
REL Stadium III
Denon DVD-3800
MF Tri-Vista 21
Better Cables Digital Coax and SE RCAs
Kimber Kable 4PR Speaker cables

Hope this helps.
Dave