http://www.audioholics.com/productreviews/avhardware/yamaharxv2500benchtestp1.php
Look at their power output into 4 ohms and 8 ohms. Also look at the damping factor.
That damping factor is almost tubelike(read: bad, when talking output impedance), and the output should at worst only go up a few watts: the fact that it goes down is evidence of either current limiting or inadequate amps. The receiver is over $1000, in which case you'd be much better off with a refurbushed 50 lbs NAD or HK which can handle 4 ohms no problem.
