Hi forum friends,I'd like to ask for your help on this "thump" issue of my vintage SE400S amp. As it has no power switch, I plug it into the switched AC outlet of its matching pre-amp SG520, according to the JBL manual. (So, when you turn on/off the preamp, SE400S is turned on/off along with it)The problem is I hear a big "thump" from the speakers (4343) when I turn on the switch of SG520, even with the volume gains of SE400S all the way down. Specifically, the woofers move outward when this thump is heard. I don't hear it when I turn it off, though. Well, I've had any such problems when I used two Crown amps in a bi-amp mode.I hear this "thump" issue has something to do with audio output DC voltage that's separated by capacitors. If that's the case, do you see any problem with the capacitors of this SE400S? Or any other issues related to other problems such as electrolytic capacitors? Please see the pic. (I've purchased it from ebay, and this conscientious seller made sure it has been gone through by a vintage technician with a clean bill of health.)Do I need to worry about any potential damage to the woofers whenever I hear this loud thump with this amp.I look forward to your feedback and opinions. Thanks much in advance.