You should forget the 'B' connection as that essentially takes power away from the 15's you like so much (in all honesty however, you should lose the 15's and get a competent sub instead. MUCH higher quality sound/bass) so since you are not running 7.1 (at least it doesn't sound like you are) you can use the 'surround back' speaker output to 'bi-amp' the 'A' speakers. In other words, leave the 15's connected to the front 'A' output and the Polks to the surround back output, go into the menu and setup for 'bi-amp'.
With all that being said, not sure what you are trying to accomplish with both sets of speakers right next to each other? You are ultimately degrading the SQ, tho of course, SQ is quite subjective, and you may be happy, but maybe you never heard how the system should properly sound? The 3310 is a damn fine AVR and honestly, you are not getting the full potential the 3310 is capable of.
Finally, don't forget to rerun Audyssey!