Magnepan 20R vs 20.1Rs

IIRC, the Radio Shack meter goes all the way up, but it's not even roughly accurate towards the top. So you could see relative changes but not absolute ones. If you do a search on the Planar Asylum, you should be able to find JBen's measurements of his MMG's with mylar front and mylar back. They're close enough to the 1.6's that I suspect the results would be similar.

I actually liked them both ways. On balance, I preferred front, because getting the high end right was more important than the better imaging from the back, but I can see that that could depend on personal preferences and your room (mine was very live when I tried it, no furnishings at all).

The Rives disc is supposed to be calibrated to the Radioshackup meter. It has 2 sets of test tones.....one at zerodb, the other tipped for the meter.

I even put a new battery in the meter without help.....

Meter between speakers aimed at center. I even tried aiming right at speaker, no help.

I suspect that the top end of the Radio Shack meters (and most inexpensive calibrated mics) is too inconsistent. If you want to make inexpensive measurements that are more meaningful, you might look into a calibrated Behringer:

http://cross-spectrum.com/measurement/calibrated_behringer.html

Parts Express is selling what looks to be the same thing with a cal file for only $40!

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=390-801