Tri-amping Qs:

You’re welcome, Gary. Glad I was helpful.

Upon re-reading my previous post, to be precise I should make a slight tweak as follows (the words shown in caps have been added):

From: "At various points in the room, if the two speakers are reproducing identical signals and again putting aside the effects of room reflections, the addition of a second speaker will cause SPL to increase by amounts ranging from minus infinity (i.e., no sound) to 6 db, depending on whether the phase relation of the two outputs at a particular vantage point causes them to sum constructively, destructively, or somewhere in between.

To: "At various points in the room, if the two speakers are reproducing identical signals and again putting aside the effects of room reflections, the addition of a second speaker will cause SPL to increase by amounts ranging from minus infinity (i.e., no sound AT SOME FREQUENCIES) to 6 db AT SOME FREQUENCIES, depending on whether the phase relation of the two outputs at a particular vantage point causes them to sum constructively, destructively, or somewhere in between.

Regards,
-- Al

iPhone-based dB meter, A-weighted 

Being A-weighted would cause a discrepancy, C-weight should be used. On my iPhone I use the SLA Lite app, I have a calibrated mic but I haven’t compared the two to see how off the iPhone is.

mzkmxcv,
Thanks for clearing that up for me. 
GP