Very cool! The Weberns and Schonbergs are probably the best sounding on-wall speakers I've heard to date. Good to hear that they are working out (aside from the height issues and bass matching).

A few random things to insert ...

I always thought that DTS did not allow for dynamic range compression (i.e., night mode), so comparing DD with DTS with the night mode turned on could exaggerate any differences that already exist between the tracks. Also, most DD tracks are encoded with a dialog normalization offset, which has the net effect of making them sound softer than the DTS track. In that case, you should raise the DD level by ~4db to level match them to the DTS track (depending on the dialog normalization setting used). Also, if you're using an older processor, you should use a test disc to level match the LFE tracks -- until about 3 or 4 years ago, it was fairly common for DD and DTS to have a 10 db difference in the default LFE levels (on my receiver, I have to boost the DTS LFE level by 10 db in order to match it to the DD LFE).

With the EX/ES engaged, you're likely hearing a "virtual" surround backfill with the surround speakers. I found that it does help with filling in the back soundfield, but on my receiver at least it tends to blunt the overall clarity somewhat compared to playing the track in native 5.1.

As far as the bass goes, I would venture to guess that your subwoofer has a room-induced interaction causing peaking and/or cancellations at certain bass frequencies. By setting the speakers to large, you're letting the other speakers fill in the frequency gaps that might exist with the subwoofer. This would be a good time to try some measuring and parametric EQing with the sub. An equalized sub will allow for tighter integration with the other speakers when set to small.