Sometimes folks need a signal tracer.and they hunt up an old Eico or Heathkit and retube and recap it but I have found something betterI believe that can be assembled on the cheap.
You should always have a test amp in the shop and preferably something that has a line in/line out function which is helpful if you're trying to isolate a problem to a particular part of your amp. One that filled the bill for me was a cheapish Dean Markley bass amp which ran me fifty bucks and a trip to beautiful downtown metropolitan Lorimor, Iowa.
I happened upon a project on a forum somewhere that consists of assembling a small project box, a 1 meg pot, a .1uf/630v or better capacitor, and a probe with some coax. These can be found in your junk box or hand assembled with the body of a BiC pen, a nail, and a sleeve.
The one I built is pictured above.
The pot is used to attenuate the signal the probe picks up so you don't fry the preamp in your shop mule amp, which is something I did with the two small Radio Shack pocket amps I had at one point. You can start at zero and slowly ramp the volume up.
Now, starting with your signal generator putting a signal in the amp under test with its output into a dummy load, you can take your tone sniffer and following the circuit see where the signal dies. At that point it is pretty easy to substitute or bridge the offending component.
I have used this rig to isolate problems and fix a number of amps with serious volume drops.
It's also of use when you're testing an amp with your scope hooked up and you can see that the volume is way lower than it ought to be.
On the subject of scopes, I'm pretty sure my Hitachi V680 died so I went and bought a Hantek digital scope for about $280, and I'm going to try it out tomorrow and see how it performs.