Hi all, I know this thread is a few years old but wanted to post for the benefit of others. I recently acquired a 661 with what I considered to be a significant hum in both channel, zero volume and all inputs, could easily be heard from 8-10' away. I had already replaced the caps in the PS (including main) and amp sections for other reasons and of course that didn't address the hum. However, after installing the wire per @olegau post #67 and rerouting the negative speaker terminal per @RonnieBiggs post #75, the hum is gone....that is, it has reduced to something so insignificant, you can only hear it if putting your head literally right up to the speaker and flipping the speaker selector on/off to compare...cannot be heard otherwise. Big thanks to these guys and all the guys and info in this thread 