Considering the 7500 doesn't have room correction, I wouldn't bother trying to get a mic. The biggest problem with using another mic than the one built for it is incorrect levels since the receiver judges spl based on a calibrated voltage reading of the mics input, which will vary depending on the mic.
Considering the receiver is a thx receiver and therefore allows a relative volume control, I would just use an spl meter. Unfortunately a majority of spl meters that are C weighted will incorrectly gauge the subwoofer volume as c weighting rolls off at lower frequencies. What I would do is set the levels for the two speakers with the spl meter at ear level, set to c weighting slow, to 75dB, then set the sub to 70dB. Set the crossover to 150 hz, Listen to a track with a familiar bass line(preferably something non electronic like dance or rap music), switch between direct mode (which disables the xover) and stereo and see if the sub sounds like it's louder or softer than the bass coming from the speakers. Adjust up and down as necessary. Once you feel you've got a good balance set the xover to about 10hz above the speakers rated - 3dB point. Go back and forth again between direct and stereo, it should sound like the sub is simply filling in the lower end where the speakers leave off, not adding tons of extra bass.
Alternatively and more precisely, you could generate a band limited pink noise using REW from the - 3dB point of your mains up to 120hz. Since the sr7500 has analog multi channel inputs, plug the output into the front l/r, then plug it into the sub input and measure the spl of both to level match them. The spl meter might roll off at the bass frequencies, but it will give you a consistent reading at the same frequencies.
The Sr7500 is a fine receiver and will deliver gobs of clean power. Enjoy your purchase.
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