[bobgpsr]

A different enclosure or different sub in the same location of the room will likely have the same response (peak/null) problems as your original SVS PC 25-31. Moving the sub around in different locations of the room might be the best bet to reduce peaks/valleys. What is wrong with your existing Radio Shack SPL meter? Especially if you hook it up to a PC sound card Aux Input and use the free REW. Moving the sub around with checking the results using REW is the first thing you should try.

Some users have found that adding another sub can fix bad room response problems. REW is invaluable to get the best results from doing this (best placement with multiple subs).

Later you could consider getting a BFD to further pull down the remaining peaks. Having a lot of fully adjustable filter channels with its parametric equalizer gives you a lot of control as compared to a one or four channel EQ built into the sub.