You should clean all the controls with deoxit for sure.
The "flexing" in your speakers is almost surely due to excessive DC offset. You need a multimeter in order to test and adjust this. I have done it in a Marantz 2235b and it was very simple, but I do own and know how to use a multimeter. If you don't have one, you can get them at Radio Shack.
You should NOT try to adjust the bias, IMO. It is possible to fry your amp if you are not careful with bias adjustment. And your issue is almost certainly DC offset.