The thing about light weight materials and deep bass is that the whole "For every action, there's an equal, and opposite reaction" thing. A light material will allow the cabinet to vibrate where it's sitting and if it's made to be as heavy and dense as possible, the cabinet will be more inert. If a weight is added to the finished cabinet, it should accomplish the same thing. Granite or solid surface material should work, it would look good and it can double as a table or plant stand.
How do you conceal the joints and corners? Plywood isn't exactly the best looking material from the edge, IMO.
Besides, if it's heavy, a second person isn't usually that hard to find and hand trucks are pretty cheap these days. I really don't see that as a problem.