From the link that you gave, your budget I gather is about $500.
The Paradigm sub itself won't really produce a lower bass that the M80 speakers that you purchased. However what it can do depending on how your setup your Receiver is to help offload some of the lower frequencies from the speakers and push that load onto the sub. Will it sound better. I have my doubts.
However you do have a second option that will work probably far more better. Take that $500 and go to some place like
http://www.canuckaudiomart.com/classifieds/43-solid-state-amplifiers/ and see what sort of external power amplifier that you can get in your area for about the same price. That way you are not counting on the rather under powered RXA-1000 amp to drive your 4ohm speakers that it can do at a push but not very loud and not to a large bass volume. The receiver that you have has the pre-out to run an external amp. I am a strong believer that and external amp for the from speakers will go a long way. The receiver if used in HT has plenty of power to drive the surrounds and probably even the centre once you offload the power hogs (L&R front). Then you can set them to Large and drive full frequency as you are no longer relying on the light weight power supply in the receiver.
If you can stretch your budget, look to spend about $600 for a 2 channel stereo amp. Check the specs. Find one that that says it's at least 4ohm if not 2ohm stable. Check the power ratings and make sure it's giving at least 75% power bump at 4ohms compared to its 8ohm spec. if you can't find a spec then pass on the amp as it's likely not worth your time or money.
