With a CD in Stereo 7 mode, you will notice more bass than with an SACD or a CD in straight stereo mode (due to the nature of the way Stereo 7 duplicates the left and right signals and then sums the bass back). It (the extra bass) is a common remark by folks using Stereo 5 and Stereo 7.

One thing that I'd probably at least try when using the split surround trick is running some test tones through the DVD player to determine if you need to trim the surround outputs down any at the player. Splitting the signal will decrease the signal strength to each channel (side and rear), but I'm not sure that the decrease will match the corresponding increase produced by the signal being reproduced by both speakers (side and rear) - if you end up with a higher overall SPL, you'll also get a little more low frequency SPL. I wouldn't expect it to be significant, but there might be an extra dB or two hiding in there somewhere.