Williarob said:

Basically, I took the L,R, LFE, LS & LR channels from the original 5.1 DTS mix, then downmixed my stereo edit to mono to replace the center track.

Nononono, you shouldn't have done that. Keep it in stereo. You'd have to redo the changes on the Center channel if you really want 5.1.

Edit: explanation: the stereo downmixed to mono and inserted as Center channel screws up the balance of the sound room as anything that was in the Left and Right channels will now be audible in the Center too. It results in a significantly reduced stereo effect, and having the surround and LFE channels is very little compensation for that. Also if the stereo version that wasn't derived from the very same 5.1 mix (and most likely wasn't), it'll not be in perfect sync with the other channels, so weird effects (reverb, flickering and such) is expected to be audible in the music parts. Not to mention that only the Center channel was modified, so when a dialog is audible both in the Center and in another channel (and it happens sometimes), one will hear both. And there's a few more artifacts.