This is one of those FYI posts... I recently recalibrated (schmalibrated) my nOrh ceramic 4.0s and SVS 20-39CS using the AVIA disc. During the frequency sweep the front left speaker had an audible buzz. Upon closer inspection I found the screws holding the woofer had loosened allowing the driver to vibrate audibly. An allen wrench quickly fixed the problem and I found all four other speakers required some retightening of their screws. This is something that had not occured to me to check, but the impact on speaker perfomance can be very dramatic.