I'm paranoid because I just bought these speakers Lol
I agreed you should play it safe. In terms of the math take a look of the following, based on the MCA amp's 29 dB voltage gain:
Pre out at 1.1 V, the MCA output will be at 120 W into 8 Ohms, or 240 W into 4 Ohms. The 4 Ohm output is irrelevant here because you are only using the Onkyo as a preamp. I am using W instead of V only because people relate to W better, for power amps anyway. Also, using W would make it easier to understand how the power amp could potentially result in increased distortions at the preamp output, though again, ignore the 4 Ohm output but focus on the 8 Ohm output because our concern for the preamp output is voltage, not current.
Based on the last two Onkyo AVRs S&V measured, my best guess is that the TX-RZ730's power amps would be clipping at or below 120 W into 8 Ohms so if you want to be sure the MCA525 stays below THD=0.1% (that is, well into clipping), then you should try to turn the volume low enough so it wouldn't exceed 1.1 V.
Your speaker's sensitivity is 93 dB, so 1.1 V from the Onkyo will get you 240 W of "clean" power; and that means near reference level from 15 ft based on two speakers.
Even if you do get some occasionally clipping from some exceptionally high peaks, it's not going to damage the RBH's tweeter because of the short duration.
If you want to have more assurance, or warning, you should contact RBH, and Onkyo as well.
Another thing you can do it, if you have a decent multimeter, you can measure the pre out voltage when playing the internal test tone. Just keep increase the volume until the meter shows 1 V. Then you can measure the SPL from you MLP with volume at 1V. If you get 105 dB C weighting slow, then you know you are good for reference level.
So the bottom line is, 1 V is very likely all you need. If that's the case then don't worry about clipping, just sit back and enjoy while waiting for the return of the AVM.
