The sound of electrolytic capacitors failing

Have your amp checked out soon. $190 is a lot, but not as much as having a bad cap take out all the power transistors, or a speaker. :)

@erik_squires A bad filter capacitor won't take out power transistors but it should damage the power *transformer*.

@atmasphere 

OH, well, even better!

One question that remains unanswered: What changes in the sound when a power cap is failing? 

Depending on the severity of the failure it can manifest in several ways
Increased hum
less bandwidth extension in either direction (this is due to the audio circuit interacting with power supply components that it normally would not) and less detail
'motorboating', a repetitive thumping sound, which may or may not vary with the volume and other controls. This problem usually occurs when more gain is present.

Sometimes the filter cap 'opens up' which is to say that its capacitance dwindles to nothing. In other cases the capacitor develops a significant lw resistance, which then draws power from the power supply (and can heat up the capacitor, which then will draw even more current). It is this failure mode which can destroy power transformers.