reciever vs seperates

Greatest improvement in sound? What are you trying to accomplish? Might want to post the rest of your system so some of the more experienced on this board can give a more informed explanation of their suggestions.

I used a denon 2800 for around 5 years and decided about 2 years ago to search for higher quality sound. Started with a seperate amp for my front L & R speakers and the difference was impressive. Cleaner highs better bass, etc. The most startling was low level coherence. We no longer had to listen at high levels to get a feel from the music. I am sure the amps in the 3800 are much better than in the 2800 but would suggest an in home audition of both the DAC and an amp for you to determine which initial direction you want to head.
The DAC (digital to analogue converter) will go between your digital source (cd player, dvd) and the reciever. Having never used an external dac I have no opinion other than to say I too have read about them and would like to give it a try.
hth
Steve

I have:

Mangepan MMGs
Denon dvd 1600
denon avr 3803
and monster cables.

Ohh .. I missed you other question.

I'd like a bigger sound stage (more projection from the speakers) and ability to listen at lower levels and still feel the music.

http://www.audioasylum.com/forums/MUG/bbs.html
Might try here for lots of info on mmg speakers. Have read that position is critical and many users tilt the speaker down toward the listening position. They are very revealing of upstream components and bad recordings. Be sure you are using a good quality recording while experimenting with placement. Would really consider trying an external amp. May give just what you are looking for after proper placement. Do try the link. There are so many there with hands on experience with these speakers that you should be able to get some good info and probably some there are using the 3802.
hth
Steve