Update: I thought I'd try an easy/cheap solution to see how much it helps before diving deeper into changing out hardware, etc. I purchased some Sorbothane washers online for $20 (enough for the front 3 speakers). I tried to calculate how much pressure the mount would put on the washer, and chose the one that matched the best, ending up with 50 durometer washers. One of the websites said 30 or 50 durometer Sorbothane was best for acoustic insulation.
I removed each mount from the wall and placed the washers (4) between the mount and the wall, with the screws going through each. I tightened down the mount in a criss-cross pattern (like torquing wheels on a car), to get even compression of the washers (website says 3-20% compression). I then reattached the speakers and ran some preliminary tests.
So, do they work? Yes, somewhat. I think I'm getting about 6-8 dB extra loss with the washers, with a total 28 dB loss from my man cave to the room above (85 dB at listening position in-room with music, 57 dB standing in the room above with hardwood floors). I'm still tweaking the compression, which changes the absorption properties, but am thinking about longer screws, as the washer is about 1/6" when compressed, so I want to make sure there's adequate surface area for the screws to hold. I may get another dB or two with tweaks. At least, now when I stand at the open basement door, I hear the sound coming up the stairway (man cave at bottom of stairway), versus through the frame/flooring.
As for aesthetics, I can barely perceive the 1/6" gap between the mount and the wall, unless I'm right up against the wall, so no issues there.