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When I first got the mic back from calibration I tried it in both directions and using the two different correction tables and saw very little differences between them. I now only use the vertical e orientation since they said it was the preferred way.
BTW I have learned when doing sweeps I wait at least 10 seconds between sweeps to make sure there is no interactions from reverberations from the previous sweep.

The UMIK-1 from miniDSP (the one I use) comes with calibration curves for the two orientations that you have mentioned.

I use TrueRTA here are pink noise and sweeps of the RAZZ and Quad ESL. The RAZZ stock tweeter is not active. I added the ESS AMT Large which gives me a much smoother response.

Volti Razz full 20 to 20k pink noise

Quad ESL full 20 t0 20k pink noise

TrueRTA sweep from 200hz to 20k
Razz

Quad

Another difference from the ECM8000 to the iMM6 would be the capsule size. The ECM8000 is probably a 1/2" capsule where the iMM6 is a 1/4" capsule. Both are omnis.

The 1/4" capsules are used in measurement mics like the Earthworks M50 with the extended 50KHz upper frequency response going down to 2Hz.

The old Wadenhome Sound calibrated mic was 1/4" as well, as is other Dayton Audio measurement mics.

I also had an ECM8000 calibrated and it had both 0deg and 90deg datasets for use. In room measurements I have seen typically use the 90deg mic orientation, where speaker measurements use the 0deg orientation.

One thing that is also discussed is a "house curve". This deals with the actual room and its acoustics.

Agree with Tom's observation that as I treated my room, flat was not as bright.

Late to this party here - but wanted to share my thoughts on the question at hand.

You don't want to confuse a speaker's power response (i.e. in-room response) with the on-axis gated response.

A typical speaker that measures flat on-axis (in an anechoic environment) will have a downward sloping response in-room. Tom P. can confirm, but I assume this largely because higher frequencies are more directional and easily absorbed (relatively) to lower frequencies - this results in lower frequencies reflected to the listening position.

If you start EQ-ing to get a flat in-room response - you will DEFINITELY find the speaker too bright.

I would discourage you from trying to EQ to get a flat in-room response.

Right on when talking about power response in the room. Unless you have a tweeter that is truly omni out to 20K like the MBL or Walsh, your power response in the room will drop off with high frequency. With my eggs, I have selected the crossover points so that the woofers and midrange are omni throughout the ranges they cover but the tweeter starts out omni and then begins to narrow around 5K. I'm enclosing the data sheet on that.

Tom