Hi Andrew, welcome to AVS!
Very nice set of speakers you've got there. Can I take you back a step? The Yamaha itself will be very low on the list of culprits for the "underwhelming" performance of the RX6's. It will not play the RX6's thunderously loud, but they should still sound good. The sound that you hear from speakers is dominated by your room characteristics and the position of the speakers in it. If you feel comfortable, please post a pic of your setup in the room. Or alternately, do up a quick plan view sketch. in the meantime, here's a few things to consider as starting point:
Recommended speaker layout ( source ).

After you've done the best you can do with your speaker setup and room, the next aspect to look at is putting that $400 (and more) towards a solid quality subwoofer. Properly set up, a good sub will provide better quality low bass throughout the room and ease the burden on your AVR and speakers.
Very nice set of speakers you've got there. Can I take you back a step? The Yamaha itself will be very low on the list of culprits for the "underwhelming" performance of the RX6's. It will not play the RX6's thunderously loud, but they should still sound good. The sound that you hear from speakers is dominated by your room characteristics and the position of the speakers in it. If you feel comfortable, please post a pic of your setup in the room. Or alternately, do up a quick plan view sketch. in the meantime, here's a few things to consider as starting point:
- Carefully examine the placement of your speakers in the room. Front speakers should be positioned as shown below. Ideally, you should place the speakers so the room is symmetrical about the viewing axis (0 degrees below). Experiment with toeing the speakers in a bit.
- Double check that the speakers are wired in correct polarity.
- Ensure you've run the Yamaha's auto setup routine correctly and checked the results.
- Go through the Yamaha's menu and look for any strange defaults (like DRC) and switch them off.
Recommended speaker layout ( source ).
After you've done the best you can do with your speaker setup and room, the next aspect to look at is putting that $400 (and more) towards a solid quality subwoofer. Properly set up, a good sub will provide better quality low bass throughout the room and ease the burden on your AVR and speakers.