If I listen to my stereo music in the reference (straight through signal) mode or direct (only bass management w/ active subwoofer) mode, what would a dedicated preamp give me?
Well, you’re using a $2500 prepro that does everything but laundry, so how much $$$ do you think went to the stereo preamp section. Answer: not much and it’ll be compromised not only by parts quality (power supply, etc.) but also by the noise created by all the digital processing, etc. So yes, using the front L/R preouts from the Emotiva to a good stereo preamp should yield significant improvements in your 2-channel performance. The preamp is one of the most critical and impactful parts of an audio system so ignore it at your peril.
Similarly a good separate DAC should also yield some significant improvements for many of the same reasons a dedicated stereo pre will. The major question is which DAC offers the sound characteristics/improvements you’re looking for, and the good news (and bad news) is there are plenty out there so take some time and read some reviews to see which ones really pique your interest sound wise. There are some really good DACs in the $1000 - $2000 price range from the likes of Denafrips, Musician, Chord, Gustard, etc. that are well worth reading up on.
Adding a good DAC and eventually a good stereo preamp should bring your 2-channel performance to a much higher level and lots to look forward to there so be excited!