Here's my six RTA-420's and one Apex 220 that I had Cross Spectrum Labs measure for me for mic compensation curves. RTA420-3 obviously has some issues!
Mic compensation curves are handy if you buy cheap mics and want some reasonable validity in your measurements. The RTA-420, Apex 220, and the DBX RTA mic all share the same form factor and likely come from the same factory in Asia, and having measured many of these mics in my Smaart classes, many exhibit a similar high frequency boost you see here, and they are all wired pin 3 hot. And no, you can't modify them to pin 2 hot without having to replace the internal circuit board.
Here's a link to Cross Spectrum Labs measurement services, no personal connection here, just a happy customer: http://www.cross-spectrum.com/
The Behringer ECM mic is a crap shoot, some of the ones I have measured are flat enough to rival test mics worth hundreds of dollars, and the rest have a similar response to the typical RTA-420. And oddly enough, a few Behringers I have measured are wired pin 2 hot, the rest pin 3 hot!
If you don't want to spend money getting your cheapie test mics measured for compensation curves, you can do it yourself with a broadband pink noise source and a known flat test mic (I use an Earthworks M30) by simply doing a transfer function between the "flat" mic and the mic under test. Not perfect by any means, but close enough to get you in the ballpark.
Keep in mind that if you compensate the response of a cheap test mic with a really wild HF response, there are unintended consequences off axis, so it's best to spend your money finding a mic that is reasonably flat to begin with. My RTA420-3 mic is only used when there's a risk that the mic may get knocked over or run over by a lift!