The receiver makes diddly difference in comparison to getting a decent sub. The Pioneer AJ sub is junk. Put the money you would have spent on a new receiver into a decent sub like a Hsu VTF-2.
If you like Polk, get the only really good center speaker they make, the LSi 706C which is available from the Polk Outlet for 1/3 of its normal retail:
LSiM706C Big Center Channel Speaker | Polk Audio™
The LSi M 706c center channel speaker features two 6.5" mid woofers, a 3.25" midrange and a 1" tweeter to disperse sound over a wide, uniform area.
The center does 60-80% of the HT output---the front mains are almost irrelevant in comparison. If you want Polk towers, their Outlet has the S50 for $200 each, which would be fine.
But if going with the $250/pr Polk S20 instead of the S50 will give you more budget to get a decent sub like that Hsu, by all means get the S20. Towers are mainly a VISUAL preference when it comes to HT unless you are specifically looking for super loud volumes.