The bass on the 60's and 80's is very similar. My room is double your size at 30ft x 31ft x 9ft and my 60's go loud enough for me with my Denon 2805. They are rated to handle a 250watt receiver, so they are no slouch. Don't get all worried about wattage, there is really not much difference between an 80watt/ch AVR versus a 100watt/ch. I would think the 60's would blow your mind in a 15' x 15' room.
I've owned a 350 and a 500. The 500 is in a league of its own, but the 350 is no slouch. I used the 350 in my large room and it did fine. Yes the 500 moves my house off of its foundation, but its worth it.
