Yeah the whole RMS / peak thing depends on the crest factor.
If you have a low crest factor then you can get away with RMS without too much audible nastyness.
This link explains crest factor quite well.
http://www.dliengineering.com/vibman/crestfactor1.htmBasically it depends on how the MPC is computing the normalise function.
If it's doing proper peak normalising it should subtract the level of the loudest sample from the maximum loudness level, then add this number to all the sample's levels.
This should not clip any levels. This is such an easy algorithm to do they should not cock it up!
A good way to check this is to normalise a series of sine waves and observe the output spectrum. You can get spectrum analysers for computers, which use the soundcard as a digitiser. With a sine waveform you get a single frequency component only. If it is clipped, you will start to get harmonics at integer frequency multiples. For example massive clipping of a sinewave to make it look more like the top has been chopped (aka sort of square-ish) will have tons of odd order harmonics, i.e. 3x, 5x, 7x the fundamental (sinewave) frequency.
If the MPC is doing RMS normalising (which I doubt) then as mentioned above, the crest factor will affect the results. You could engineer a sample to have a very high crest factor and try normalising it then transfer it to a computer for analysis. If you could keep it in the digital domain (i.e. use the SPDIFs) then you should have a high level of accuracy. Decent wave editors allow you to measure the individual sample amplitudes. Since normalise is a linear function it should scale all samples equally. By measuring, say, positive and negative peak sample levels before and after normalisation you should highlight any odd behaviour.
Finally, might I suggest it _could_ be psychoacoustic. When you next get one of these dodgy samples, record it into a computer and check for any plateaus in level on the peaks (indicating digital clipping), if you can't see them but can hear them, then maybe it's some odd human brain behaviour. (like this...
http://www.youtube.com/watch?v=QbKw0_v2clo)