This mostly happens if you use high excursion speakers, which is one of the reasons I have posted that the circuit is not as usable with "real" speakers, but if you notice, the speaker may jump, but the cone stays in control as opposed to what would happen if you cranked up the bass control to the point where you have speaker fart/bottoming out. This has to do with the harmonic distortion generated by that circuit. It's part of the illusion, but is exaggerated by "real" speakers.
There was an issue with the Aux in grounding on some. They would function fine, but would be noisier than normal. It could be corrected by any Peavey service center, the factory, or me. The fix is very simple and could be done by pretty much any tech.
The earliest amps have a serial number that starts with 0DBFI followed by several other numbers. If you find one with that prefix, look at the last 3 digits. If the last 3 digits are higher than 100, it will not have the issue. All others are okay.