Regarding the volume control.
Disassemble and clean / recondition the one you have.
I have a 607dr and there are some annoying engineering decisions that make servicing the controls a bit harder than it should be, the more deeply seated, more difficult to access, selector sliders cause a lot of sound problems once they get dirty.
The volume controls in front are easy to access and remove. Here is how Kale does it... (I followed his example and it was very much worth the effort.)
The retaining ring will need to be replaced with a new one.
Clean away all the brown stuff, reassemble, and it will be like new.