Subwoofer Thump

Is there a way to avoid subwoofer thump when turning off BP50 preamp.

Yes, there is a way. Turn on the BP50 preamp, then the subwoofer.
That will fix your problem.

You are welcome.

Victor thanks for the response. The problem is when I turn the BP50 off and get a sub thump. Like most people I leave my sub on auto on, yes I could go around to the back of the sub and turn it off first to avoid the thump but that would mean pulling the sub out from its location to do this. I have had other amps with a sub connected and never had this problem before.

I understand. May I suggest you to get a Smart Plug like the TP-Link KASA.
With that you do not need to go to the back of the Sub for turn it on/off.