Hi, and welcome to the forum!
First, the Polks should pair just fine with that Denon. I am one of the people that believe that you don't need to pair particular brands of electronics with particular brands of speakers. The Denon (and most modern receivers) has an auto set-up routine that will do some equalization for you, and because those set-up algorithms differ between some companies, the end result is that the systems will sound a bit different with the different brands - but that is more a function of your room (IMO) than the brand names.
My last experience with listening to Polks was over ten years ago, so I'm in no position to comment on those models. Several people here have recommended other brands of subs above Polk, but I know people who are happy with their Polk subs. The boominess could have been a function of the set-up and not some inherent aspect of the system to which you were listening.
Adam