If you have good ears, one quick way is to turn the volume up one or two clicks at a time until you hear distortions. You can simply play something from your favorite collections that you know has excellent recording quality that you know you cannot hear any distortions at your normal listening levels, then if you start hearing the slightest sign of distortions during the peaks such as a cymbal crash, hold it right there and listen to the same peak(s) a few more time to make sure you find the volume position where you first detected such distortions. Now if the volume is at say, 0 on the relative scale, and you normally listen to this same piece of music at -10, then you have 10 dB of headroom. This is just a quick and easy way, you can do better if you have the proper measuring equipment and/or software. Also, this assumes the bottleneck is the AVR, not your speakers.
Yes, your "Part of the answer...." is correct, but as TLGGuy said, just turn the volume down!! In other words, the volume position will be much lower for the same sound pressure level, if you flip the scenario from feeding a 1.2 V sensitivity power amp with a 0.7 V rated preamp from feeding a 0.7 V rated sensitivity power amp with a 1.2 V rated output preamp. So that is a non issue, but there is a practical limit, though power amps with sensitivity higher (i.e. lower specified voltage, or higher specified dB) are rare.
You are not alone, I wish they (for AVRs, AVPs and integrated amps) would all specify something like:
Pre-out: 1.2 V RMS, CFP-BW, at <0.01% THD, 3.0 V RMS Maximum at 0.05% THD, >500 ohms.
That's not a lot to ask, and Yamaha did come close to this, but they did it for their AVP CX-A5000 series only.
It did seem to vary between models, and likely the test protocols by different labs could have been different too. Also, keep in mind voltage stated in RMS, peak and peak to peak will obviously be different because:
For sine waves (that's used in those tests),
Peak = RMS X √ 2, or RMS X 1.414
Peak to peak = RMS X 2
If they don't specify RMS, I typically would assume they mean RMS if it is from a reputable manufacturer such as D&M.
For those speakers, in a medium size room you will be fine with the SR6012. It depends mainly on how far you sit, if you sit further than 10-12 ft and you listen at or near reference level, the SR6012 won't have much headroom left, may even struggle.
The AVR-X4400H would have given you a little more watts/$ for the same money but regardless, to make a 3 dB difference you need double the power. If you have one or two powerful subwoofers, and set crossovers to 80 or even 100 Hz, you may only need to add 3 X M2200 or a Monolith 3X200 W for the front speakers.
No idea what .825mVa is, you probably heard him/her wrong.
Those specs are typically approximately correct, to a few decimal points. Based on my own calculations and testing, the gain is closer to 27.5 dB or even higher, using the RCA inputs so that's accurate enough I guess. I suspect the M2200 will level match very well with D&M AVRs (including your SR6012). It matches my Denon AVR-X4400H and Marantz AV8801 to within 1 dB on level tests using a spl meter.
Good observation, the 225 WPC is for one channel, with two channel it will be close, but with 3 channel driven it probably would drop to 190-200 W, just my educated guess, and that's close enough. If you are in Canada, the MCA 325 would be the one to get. If in the US, I would go for the M2200. I just got mine for $299 when it was on sale. Regular price is $380, but I am sure the $299 sale will be back soon enough.
