Well if you plan to add a sub I probably wouldn't go to A5s. Use that money towards the subwoofer instead. I believe the A5s only get you another 15hz or so of extension so not really worth it if you want more low end.
STF-1 Subwoofer
M8 Powered Subwoofer
either of these options would put you under $300 and still give you good bass. It's hard to recommend a sub for less than about $300 because the quality tends to jump significantly in the $3-500 range.
I have the Z-5500 as well and while that sub is halfway decent, a good quality sub will sound so much better and tighter.