Greetings,
Without knowing which dvd player you have, it would difficult to guess the issue. Be sure the dvd is coded for dts, too. I'd wager it would be the dvd player, not the receiver. I've owned the htr 5660, and I believe that it is the distribution equivalent of the rxv-640, since there was no htr equivalent of the rxv-740. Also, even though it is spec'ed at 85 wpc, Yamaha confirmed to me on 2 occasions that it was in fact rated at 90 wpc. The specs were apparently written up before it actually went into production where it gained a few more watts. If you have the owners manual, you'll notice that the specs published in the front differ from those in the back.
b.
