best dvd player under $1000 new or used

Why would you want to upgrade the Denon? That is tough machine to beat. Is yours a MKII? If you want a multi disc player try the Pioneer Elite 47Ai at 999.00 in most stores.
It plays SACD Stereo and Multi Channel, DVD Audio and DVD Vidio as well as redbook CD's It also has a 108 x 54 video chipset. For alittle more get the Marantz DV8300 a much better d/a audio chipset and the same pioneer video section.

The Denon 5000 can be had new sometimes on ebay for $650. No progressive scan, and it won't play CD-R or CD-RW or MP3. Has HDCD. Make sure it is plugged and turned on for three days before you listen to it, otherwise it will sound harsh and thin. It has a standby mode, so leave it on all the time for best sound. Sounds as goods as the smart ART DI/O with the power supply and power cord up grade. Received only one good review. Must be the AL 24 processing.

For players under 1K:

Philips DVDQ50 if you want picture.
Philips SACD1000 if you want best HT sound (use the enclosed decoder, with the HDAM output stage)
Sony 9000 if you want picture & sound comprimising good.