Tvad...My listening is mostly multichannel. Why don't you beg, borrow or steal an extra amp and pair of speakers and give it a try. (If you can russle up a center channel also that would be better).
All DVDA can be played in 2-channel mode.
My measure of audio quality includes the spatial effects of multichannel, so my "best" discs may not be yours.
Try TACET DVD D 107 Mozart Flute Concertos (available from elusivedisc.com). This is nice clean audio, good music, and dramatic when played in multichannel mode. (This disc is the reason that I upgraded my rear speakers).
Try AIX 80008 John McEuen and Jimmy Ibbotson, Nitty Gritty Surround (Bluegrass). This disc illustrates the great flexibility of the DVD media. If you have a HT system, or can visit a friend who has one, try this disc on that system also. In addition to the DVDA program, it includes programs that can be played on a DVD-Video machine. The DTS programs are very acceptable, sonicly.
WARNING...THIS EXERCISE MAY BE HAZARDOUS TO YOUR BANK ACCOUNT.