subwoofer thump on player

albums are all stored vertically

with the sub I get the once a rotation light subwoofer thump

at low volume levels

will a record clamp or weight help

still a cd guy mostly
Well that's 1 of many reasons why I gave up on vinyl. Does it happen on all albums? It could it be an off center hole or warp if the thump is only 1 time per revolution. I know I'm not much help but that was a severe problem I encountered all too much back in the day when I had a vinyl rig.
Get a low pass filter
Could you swap cartridges if your still using the B&O.I was just wondering if its suspension is getting weak.I can't remember if they had a tracking force adjustment.
Turn off your sub. Vinyl is not a medium that does much below the 20-30Hz range. This could be bearing rumble or drive system issue. Either way, I would not expect much good from that table in the down low area.
If a thump occurs on multiple LP's precisely once-per-revolution, the cause must be in the rotating parts of the table.

It might be an uneven spot in the platter or matt, a worn spot on the bearing, an anomaly in the drive system or motor or anything else that rotates. The only way to know is to investigate the various possibilities, which only someone with access to the table can do. Consider this an opportunity to get to know your rig a little more intimately.

That said, from a practical perspective I second Dan_Ed's advice. Whether you find the problem or not, turn off the sub (which is a free way of providing a LF filter!). While some LP's do have notes below the 35-40 Hz rolloff of your Thiel's, the B&O rig won't reproduce them well enough to be worth worrying about. When spinning vinyl, your sub is likely to produce more mud than music.