I have an old Peavey 2 x 15 cabinet that I loaded with two cast frame 400 watt EV 15's that look like this; http://images.search.yahoo.com/imag...hmv&sigi=1175qs5fr&.crumb=eAIKtGLT/4B&fr=ipad I just recently sealed up the ports on this cabinet to see if it would tighten up the sound. Each speaker has it's own compartment and does not have the air pressure influence from the other speaker. Here's the twist; when I play an open E or A string, the speakers vibrate at the frequency being played, but one slowly moves outward from its frame and the other pulls inward deeper into its frame. Each are still vibrating at frequency. I've checked and re-checked the polarity by using a 9 volt battery and touching the batt. terminals to the tip and ring / ground of a speaker cable plugged in the cabinet. Both speakers always move in the same direction. Is the slow drift of the cones in opposite direction the result of one compartment not being sealed as good as the other? What are the other possibilities?