Where did my very low end air go?

So you are basically processing your Dolby Dig/ DTS in the Oppo, and running out, analog, to your Bel Canto - to simply act as a volume control and switcher.
My experience has been that you'll get a flatter, less dynamic sound processing this way for digital movie source material. I think if you simply back track on your setup, and go back to connecting digital to a fully digital pre/pro- and process internally in the pre/pro - you'll find much more dynamics and power again!
If you haven't changed anything in your setup, than this should be a simple solve.
You of course should always measure for response, phase, volume levels, etc anyway, to know where you're at, and to dial in any system. (I presume 98% of all people don't mess with any of this beyond what auto-EQ's process. They could have their subs and speaker/seats sitting in giant acoustic holes in the response, and they'd never know it).
But to simplify, you should just switch back to the old connection method and see what that yeilds you. That is, if it's important to you in really knowing what happened.
I would.

Yes I would like to know. I sold the Krell HTS. I can not go back without buying new equipment. I do like the midrange better in the Bel Canto Pre6 better than the Krell. There are not a lot of movies with the very low air. I have been so busy with my 2 channel stereo rig that I have not been using my video system. I thought of one more thing. The Krell did not process the latest formats which the Oppo 83SE can. Do different formats carry more low end air than others?

I would try to compare fundamental setup options first, since you just changed this out in your system. You went from a direct digital connection, processing in the pre/pro, to an analog connection from sources, processing in the Oppo. You should compare these two paths in your system, side by side.
You could easily buy a used piece, insert into your system, and compare. You can then sell what you don't want. You can also go down to local Best Buy, buy a descent AV receiver, try that in your system, and return if it doesn't work out (hint hint). You'll then be able to compare apples to oranges, and trouble shoot your setup options.
You'll at least then know what you have going, and the fundamental differences in connection and processing options. Because, as of right now, you've switching things up quite a bit on the processing end.