There is not much output from the sub. I have spent almost 2 weeks doing the sub crawl. I have ran Audyssey 4 different times. The first time I used the parameters listed in the box as a starting point, and the sub was barely audible. I followed some suggestions on here and started the sub at 75db before running Audyssey, then bumped it up with the receiver 8 to 10 db, and this made a difference, but not a ton. Still has far less output the the SVS PB 2000 it is replacing. I know they are not going to sound the Same and the Monolith should have a flatter response but anything above 35-40 hz sounds hollow, almost anemic.
Last time I turned all THX settings off and added a Y Adapter to the inputs on the sub. Turned the gain knob to above +6 or about 75-80% on the back and it did come alive a bit, receiver sub setting was +4. Played about 15 minutes of Ready Player One and probably -15 on Marantz volume, and it sounded closer to what I was expecting, but after 15 minutes there was a smell from the sub, so I turned it off.
I have adjusted the phase, all eq settings. Currently all THX is off and EQ set up through the Marantz. Fronts left to small and at 80hz, LFE +Main turned off, Dynamic EQ turned off. I almost need a spreadsheet to keep track of everything I have tried, and I still want to most my SVS back downstairs
That is a lot of things to try. It definitely sounds like you should RMA it.