rough method for clipping detection

I don't think that's correct.  Your voltmeter is already giving you averaged readings, not peak to peak.  If it was peak to peak your test would be correct actually because as soon as the voltage stopped going up you'd be clipping and you'd be able to set your gains.  Unfortunately that's not how it works.  When it clips it goes to max/min voltage and stays there (square wave) and then rapidly switches.  Here ya go: http://www.geofex.com/effxfaq/distn101.htm. Much easier to understand with a picture.  Since the voltmeter sees longer max and min values it averages them and gives a higher reading.  Thus once again you're not going to be able to tell when your amplifier goes into clipping.  Just can't do it.

Also you do not need to necessarily give max. rms power not to mention exceeding it, to get your sub to reach xmax.   That is very dependent on your enclosure and frequencies being played.  Since you're competeing in SPL i'm sure you're aware of the importace of the enclosure, tuning frequencies, matching your vehicle's peaks and all that?

I appreciate the link.  I'll look into it after I grab a shower.

I agree with what your saying.  I'll have to spend a little more time with this to try and fully understand the concept.  Those graphs should help.

I'm tuning for a 70 Hz peak.  I was building to the 70 Hz resonance frequency, but after analyzing the curves in a program, I determined that that was not the best course of action.  If you have any advice, I'm all ears.