That woofer excursion thump is very likely subsonic bass distortion. There are warps in vinyl records that create the stylus to pick up a LONG signal and your amp recreates it, sending the signal to the speaker. The amp controls the movement of the speaker magnet - hence the distortion and "thump" as the woofer magnet reaches it's travel limit. That is MURDER to speakers and draws LARGE current to your amp. Both damaging to equipment.
You're not dropping your tonearm on the vinyl suddently are you?
You're not dropping your tonearm on the vinyl suddently are you?