I was listening to Andrea Bocelli: Under the Desert Sky in DTS, but only got stereo. I switched over to a Clapton DVD and DTS worked fine. I then switched back over to the Bocelli DVD and it started to play in DTS. When I was in stereo the first time, I tried everything to get it to go into DTS, but nothing would work. It finally worked when I switched discs. Is this normal?

It's not something I recall running into. Any chance that the Bocelli disc had a two-channel PCM track as default, or that the 990 was set to a "stereo" mode that caused it to downmix the DTS?

Yes is does have a PCM track which is default. I always chose DTS, but this time I noticed that there was no surround. So I thought I must not have set it to DTS, maybe I fat fingured it, but going back into the menu and selecting DTS again both on the disc and in the 990 would not free it from its death grip on PCM. I needed to play a different DVD with DTS to get it to comply. Once in PCM there is no going back, it seems.