I'm not sure what you mean by a "more precise instrument", JB. I don't think that measuring SPL in db is particularly esoteric technology anymore.
Tom, precision is pretty much a function of cost. When i was in the Navy we had some SPL meters that were a couple thousand dollars each, and had to be calibrated every quarter... As J.B. said, with those meters 75DB, was EXACTLY 75db, measured to the 1000'th..
However, the precision of an AVR is a function of the quality of the mic being used... even the audyssey pro kit only costs 500$. So, how accurate can that mic be? Also for HT stuff how accurate does the mic need to be? I mean, any of the mic's used are most likely WAY more sensitive than the average ear... So, i would propose that a Radio Shack meter would be more than sufficient..