Weird overtones from a speaker cab?

[fireincairo]

Ok, so I've got this Peavey 4x10TX cab I just bought yesterday. At lower volumes, it seemed to sound fine. When I got it home and ran it at high volumes I began to notice some overtones on the E and A strings.

It's almost like there is a double note being played, one of which is not in key. Could this be the speakers or tuning of the cab? I'm running a Peavey Butcher 120 watt tube head into it and a couple of J/P basses into it.

 

[fireincairo]

I think the cab couldn't handle the lower frequencies I was pumping into it. Now that I've backed off the bass EQ and switched from P neck pickup to J bridge pickup all the overtones are gone.

It sounds pretty darn good, too, but now it seems I won't be able to use my P neck pickups with this amp.