Hi and Welcome,
(Have I been around long enough to say that now?)
I wouldn't worry too much about the older sub. My HT is in the basement but I have a pair of M60s in my living room. It's not as long as yours at 16x15 but the ceilings are much higher, going right to the roof of my two story. I also have an open dining room on one side leading to the kitchen. I can see my speakers from over the dividing bar between.
Anyway, what I wanted to say was that the M60s sounded surprisingly deep in that bigger space and seem to overcome the echo-like sound my old HK bookshelves made before I got them set up. When I got my EP500 for the HT room, I moved my old beater Sony HTIB sub up from the basement to join them. It did round out the bass a bit in that big air space and everyone comments on how well they sound.
Too shorten my alwyas too long post... The M60s hardly needed any help at all to fill the open space. You will be very happy with them just with the HSU until you get the urge to upgrade.
buttttt, if you can afford a new sub, my other m60s in the basement with the EP500 and other goodies really blow things away for bass.
