Certainly a lot to Fender amp have this cap in series as the Reservior Cap. In amps generally hum will increase and a noticable loosening of the lower notes. The only time I have ever use an
equivalent series resistance (ESR) meter is problems with switch mode power supplies on a piece of machinery, amps are not really precision. All they do is overheat and bulge and sometimes if left sitting the will explide when I flip the switch.

I play with radios from the 30's often.
Am I to assume that there is no correlation between esr and the condition of capacitors of higher uf and voltage? No, do not assume this. Simply refer to a datasheet for the brand and part of cap you have. Lately I have see a lot of posts on ESR meters, which are nice to use since you can test withing the circuit. I just have never used on on an amp simply becaue I am generally repairing one in this situation. If the Electrolytics are 10 years old or more, I just replace them while servicing. It just makes sense while I have the chassis out. I don't think I would ever use old electrylytics if building a new amp.
I not sure, but I think the gray areas on the chart Jojokeo posted might mean the value are the same as the above number. In other owrds, this may be as low as it goes.
Then again, I may be full of crap and if so one of the folks will stop by and let us know.