Never heard of Millennium speakers. Where did you get these? Did you buy them from a white van or someone you met on the street?
Sounds to me like one of the phony brand names the "white van" bogus operaters use to sell speakers that supposedly are $1500 a pair and overstocked. They all are junk. Hope you didn't get caught up in one of these purchases.
If that is the case then best you start over. Buy a quality 5.1 set of speakers all from the same brand. You have a quality DVD and super quality receiver.
Lets us know more about these speakers, where you bought them, what is the address of the manufacturer, do you have a owner's manual?
I don't have high-end surround speaker, so I was just wondering will changing these make a huge difference? I have Millennium MTR-2523's for front L/R which are very good. With Dolby Digital I find the sound not to be enveloping, so what can I do to make my system sound better, I have a Denon 1920 DVD-Player and Pioneer Elite 74txvi (which I LOVE!!!!!) reciever.