In reply to:

What is the best way to callibrate two subs?
Would it be one at a time or both together?
I have the callibration disc and the SPL meter.
The speakers in question would be an EP500 feeding
into a Polk PWS450. I'm not using a Y connector, just
line out/line in.

Hi Ken. Personally, I would stack the two in a corner (PSW450 on top of the EP500) and set the PSW at the 12 o'clock position and the EP500 to the 9-10 o'clock position. Next, calibrate with reference disc test tones (most likely 75db). By stacking the two in the corner, you should get about a 6db boost. I did this in the past with two M&K MX350 subs (as suggested by Ken of M&K) and it's much more smooth and deeper base. Good luck, and let us know how it turns out.
Mini Rock's