Radio Shack's SPL meter is fine, but an SPL meter is not needed for balance, just for overall output (like channel balancing multichannel or capturing frequency response anomalies like room modes or nulls). For left/right channel balance simply use a mono signal, with correct polarity, and make sure the sound is coming from dead center. If not, your room, speaker or source could be the problem (mostly room issues if speakers are equidistant). So, try:
* toe in one speaker slightly, especially if one speaker is closer to a wall structure than the other speaker
* use balance control on preamp to shift image (easiest fix but doesn't address real problem...but who cares

)
* move some things around in room and try again