Hey Doug....Thanks for stopping by.
I don't think it matters much as far as carpet on the floor of the theater. With either BOSS platform, the key is to isolate the entire platform from the floor as much as possible. The carpet probably helps some with the isolation, but the soft rubber (20-40 durometer) between the platform and the floor is what really does the trick. Having said that, I found carpet on the BOSS platform itself definitely helps, especially where the chair/couch legs rest on the platform. I found without carpet, my furniture pieces would move a couple inches after spirited listening sessions.....the carpet helps keep the furniture in place....lol. FWIW, from an acoustics perspective, you probably want something on your theater floor to minimize slap echo....rugs, carpet, etc.
On my big riser BOSS, you are correct, I was only able to get 2 of the rubber isolators under one end of the riser (1 isolator under each corner of the riser farthest away from the columns). 4 isolators (one under each corner of the riser) would have been preferred, but the corners underneath each column couldn't be jacked up because the columns are connected directly to the riser. It would have been major surgery to modify the columns to accommodate the riser being raised 1/2" or so on that side. Plus, I found just getting the one end of the riser lifted off the floor with the rubber isolators delivered the same effects as the front mini-riser which is totally isolated from the floor. This is because my back riser is so stiff (it's constructed of 2x12's with 3/4" plywood on top). It doesn't bend at all. Even at mid-span with people on the platform, I can still fit a finger between the riser and the floor.
The key is to isolate the entire platform as much as possible from the floor. So, the additional isolators at mid-span would only start to couple the platform back to the floor again since I would be filling the air gap that's there today with rubber.....Hope this makes sense. Essentially, the only part of the back riser platform that is touching the floor is the one side of it under the columns...every other part of the bottom of the plaform is in the air.
The big riser is totally open on the bottom and closed all around it's perimeter. You want the bottom of your BOSS risers, to be open so they won't act as cabinets and limit the excursion of the JBL's. You want as much excursion from the JBL's as possible. There's also other advantages to the open bottom (technically called an open baffle)....namely SPL cancellation which makes integration of the BOSS into your existing speaker/sub system a breeze. No need to include the BOSS in your room measurements and having to re-calibrate since there is very little SPL from the BOSS. Just add the BOSS and follow the 80/80/180 or 80/80/0 rules and enjoy.
For power, I would stay close to the 80 watts per driver. So, for your 5x150W Marantz, I would wire the 2 front row JBL's in series for 8 ohms and run the front row on one channel of the Marantz for ~75 watts per JBL. For the back row, I would use just 2 of the JBL's wired in series the same as the front row and use another channel on the Marantz for ~75 watts per JBL. Adding a third JBL to the back row will bump your impedance up to 12 ohms and your Marantz will only provide ~100 watts per channel at 12 ohms. That would only be ~33 watts per JBL....not enough power to get the cone excursion you need for the BOSS to really deliver the smiles.
If you find the 2 JBL's in your back riser isn't enough (I think you will like it a lot), then you could always add 2 more JBL's back there and run those off a third channel of your Marantz.
Hope this helps.