Choosing a capacitor to buy (Mullard tropical fish substitute)
It's tricky because unless you find an identical replacement capacitor (its specifications - manufacturer not important) the choice depends on the circuit. If you're really lucky, the circuit isn't fussy at all. Worst case scenario is the circuit relies on obscure properties, and/or relies on inferior qualities.

A capacitor has many properties. The main one, obviously, is capacitance. Other important qualities are maximum voltage it can withstand, and capacitance variability between parts. Other things like capacitance stability over temperature etc, and then there's the non-ideal capacitor properties. In theory an ideal capacitor has zero resistance and inductance; that is, it's purely capacitance. But it's obviously not the case in the real world, and sometimes that matters.

First find a datasheet for the capacitor you wish to replace. Then compare with datasheet for possible replacements. The closer the match on all properties, the better. It's as "simple" as that. 

Chances are it's not hugely fussy. I'd aim for as many of these as you can:


Exactly the same capacitance
Equal or better capacitance tolerance (eg +/- 10%)
Equal or higher voltage, but not too much higher
Equal or lower ESR (Equivalent series resistance)
Equal or lower ESL (Equivalent series inductance)
Equal or lower leakage rate
Equal or better temperature coefficient
Identical capacitor type if possible - ie resin dipped polyester - but perhaps "greenie" metalized mylar  will do.


And of course, size. Make sure it fits!

If you can get hold of the circuit diagram for your amp, it might provide insight for eliminating or easing many of the above constraints.