I agree with you 100% on this.
Not necessarily as it depends, it would appear that Rega emphasized the 760 mV figure because it's build in power amp has a gain of 31.5 dB, that means at 760 mV input, the power amp section will be at rated output of around 105W. It is lower than many amps mainly because it's relatively high gain (31.5 Vs THX's 29 dB) and lower output level.
No idea why your would "doubt it".. Of course the input can exceed the sensitivity of the Arcam P49. As you know the "nominal" sensitivity of the P49 is only 1.15V. Even the recently tested AVR-X3300W can output 4.5V unclipped.
http://www.audioholics.com/av-receiver-reviews/denon-avr-x3300w-1/measurements
As Gene said: "A preamp output of less than 2Vrms is unacceptable in my book."
Rega did not specify it's maximum pre-amp output level, but I sure hope a relatively expensive unit like that can output more than 2V, and that would have no trouble driving the Arcam to exceed it's rated output. Note: the 760 mV was not stated as the "maximum" output, it was simply stated as the level when the Rega will be at rated output.
No need for an oscilloscope for this, I will still use my Fluke 87V multi-meter even if I have a good scope on hand. First of all, I wasn't measuring peak, but true RMS V max and average only. There was no need to, but I actually always measure peaks as well just for knowledge, the 87V can measure transients as fast as 250 micro seconds, not that fast but fast none the less. You can see the features/specs of my mm in the link below:
http://en-us.fluke.com/products/digital-multimeters/fluke-87v-digital-multimeter.html#features
By the way, one of my power amp, the Marantz SM-7, has analog watt meters. I don't want to risk smoking it, otherwise I could do a demo and show you the photos.
As an example, I would simply hook up my Denon DVD3910 that has output voltage of approx. 2.5V or higher depending on the source CD, DVDA, SACD etc. That should be high enough to drive one of my preamp to output >2V with vol at 12.
I understand what you are getting at. The OP would have to risk hearing damage to drive his Rega/Arcam setup to trigger a thermal shutdown or high current (e.g. short circuit) protection shutdown, but the point is, it is possible. Let's say he has a large room of 20'WX30'LX15'H and he sits 16'X20' from the speakers. Now you do the math or use the more conservative Crownaudio spl calculator, you will see that it is not hard to get the Arcam to exceed it's 200 WPC into 8 ohm, both channel driven or 400 WPC into 4 ohm, one single channel driven ratings.
Keep in mind as TLS Guy pointed out, the speakers low impedance level coincided with a relatively high phase angle. That alone should not be an issue, but if all the stars align, and if the OP listen at above reference level, say 90-95 dB at 16 to 20' from the speakers, then things could get ugly.
I only suggested measuring the spl from a given distance so we could
rule out the possibility that the OP may be listening way too loud sitting far away in a large room (if he is in a large room). It is so easy to do to rule this unlikely scenario out so why not?
So you misunderstood me all along. I am not suggesting this as a way to find the problem. I am suggesting doing it to
rule out a potential probable cause. Again, it is easy to first rule out the listening too loud while sitting too far, and driving speakers with impedance dips over a range that coincides with a somewhat difficult phase angle etc etc scenario. After proving this to be negative, then the OP should be on firmer ground to ask for an exchange. I hope I made it clear this time as I think it is time to move on.
Edit: for calculation purposes, I would use not use the 91 dB/1W/1M for sensitivity because it is specified on /2.83V/1M basis.. 89 dB/1W/1M would likely be more realistic because looking at the graphs TLSGuy posted, the 8 ohm nominal spec seems too optimistic. I disagree with TLSG's 4 ohm assessment but I would go as low as 5 to 6 ohms for power calculations or online calculators that are based on dB/1W/1M sensitivity specs.