Hohlraum, welcome. You can be sure that you have an excellent sub in the EP500, as you'll find when it's properly set up. The first point, as Randy mentioned, is that setting the "crossover" control on the back of the sub to the same number doesn't result in "matching", but more likely a conflict. It should be set to bypass or turned all the way up so that it doesn't get in the way of the crossover that your 5300 is doing at 80Hz.
Secondly, unless your room is very large, you may find that setting the trim even to full(as I found to my surprise with my EP500)may result in a smoother overall room response.
As has already been mentioned, you should calibrate the speaker and sub levels, and this shouldn't be done by ear. Either use the auto-calibration system of your 5300 or a separate SPL meter which you'd use to make level adjustments manually. Either way should give good results.
