The thing that might give you a better tone from a different receiver would be the auto EQ feature. Audyssey, ypao, and mcacc being the most popular. They do however use different approaches, and mostly give different results as such. If you use direct/pure direct they will sound the same.
For the 505, I have a lot of experience with that sub. While it does put out enough power for some people and their rooms, I’ve found that it’s not very articulate, and loses output much too high(about 25hz in my room). I’ve also found it to not sound very good up in the 80-120hz range. IMO, upgrading that would give “a better tone” that you’re looking for.
It’s kind of a tricky concept, but bass is much more than just output. The sound should remain linear(true to the signal) and controlled
(well damped without ringing and overhang). IMO, said sub should also be able to reach down to 20hz. While the last point is arguable, there are many fantastic soundtracks that go low enough to make it worthwhile.
I understand why you’d be reticent to consider a new sub/s, but once you know, you can never go back.