No offense, but renting a car for RMAF seems to me a waste of money for 2 primary reasons:
(1) The RMAF speaker show itself is more than big enough to occupy your time Friday through Sunday non-stop. You could spend just 10 minutes per room every day (at least 10 minutes waiting per room in cases - as you'll inevitably have to wait for someone else to finish listening) and still not see every room. And I'm not even including the Head-Fi room, if you're planning on listening to headphone gear.
(2) Unless you know exactly where you're going or will be offering transportation to fellow Head-Fiers, I don't see why you'd need a car. There are restaurants within walking distance. The area around the DTC consists mostly of tech companies, restaurants, and stores, and it's bounded by two highways. The nearest "tourist attraction" area is downtown Denver, which is 15-20 minutes up the highway, and then you have to deal with parking the rental car, which is very expensive in Denver. The RTD light rail station next to the hotel (Belleview Station) can get you to downtown more easily (though it does take a bit longer), and it'll take you directly to the heart of downtown, the 16th Street Mall.
(3) You'd also have to contend with the limited parking at the Marriott DTC. Every year the parking fills up and lots of people end up having to park elsewhere later on in the day.
The only valid reason I could see someone having for a rental car would be to explore beyond Denver for whatever reason (like going up into the mountains being the best example I can think of), but if you're not planning on doing that, I'd recommend cancelling your rental car. The entire area around the DTC for 30-45 minutes in every direction consists solely of residential suburbs (with the exception of downtown Denver obviously) - really not much to check out in that aspect.
Oh and I know of at least 4 CO locals who'll be there with their cars - if you really want to get out somewhere, why not ask for a ride?
