Any chance you can stretch the budget a little...?
http://www.bkelec.com/HiFi/Sub_Woofers/XLS200.htm
Based on a very good Peerless drive unit, of the subs I've built using the XLS drivers they can take some punishment!
With speakers setup is everything, have a play with Room EQ Wizard [
http://www.hometheatershack.com/roomeq/] while you move it about to reduce room modes. Ideally you need a parametric equaliser with any sub that has decent low end extension, either a seperate one (I use the good old Behringer Feedback Destroyer Pro) or the EQ quite a few modern receivers have now is better than nothing. Looking at the Sony website it does'nt look like your amp does any EQ aside from tone controls
A decent subwoofer is essential IMO even in stereo, you only have one low end to EQ and it takes the heavy lifting away from the rest of the speakers/amps giving you much more headroom and lower distortion. I've got proper floorstanders for fronts which can get down into the mid 30Hz range in room but I still cut everything below 60Hz and send it to my sub
