Pass X150 is producing "Pop" during power-down

Hello all,

I have been experiencing a thump or "pop" from my recently purchased X150 (5yr old) when I power down the amp using the front button. The previous owner didn't experience this. I am using the single-ended RCA input from my receiver for now. Also, I notice all the woofers of the speakers retract (ie get sucked in or move backward) upon the power on and stay retracted until I power it down which gives the thump or pop .

If I physically disconnected the preamp from the amp, I get the "pop" not only during power-down but also during power-on.

Interestingly, the turn off thump and woofer retraction phenomenon disappear totally if I remove the XLR pin1 to pin3 jumper to pretend having balanced input without physically having XLR cable plugged it. (For those who doesn't own this amp, there is a jumper connecting pin1 to pin3 of the XLR input if you are using single-ended input).

Pls excuse my long description. My questions are 1) Are these normal things to expect if I am using single-ended input? I am worried since the bass response seems compromised.

Thanks in advance for your kind input.

Kelvin