I've told my Frontman 65r has a real reverb tank (accutronics). But it's almost imperceptible, even with the reverb knob at 10. Is that normal? Also, mid scoop for the drive channel and treble/bass for the clean channel have almost no effect on the sound. Is that also normal?

http://cie-wc.edu/Series_Parallel_9_14.pdf
Not recommended.
you could add 3 more 8 ohm speakers in series / parallel wiring and keep an 8 ohm load to the amp, or
2x 4 ohm speakers in series, or
2x 16 ohm speakers in parallel

After ruling out guitar wiring or bad cables...
Try the 2nd input on the amp
also
Is it on both channels, or just one?

This sounds like one of this nasty problems with the solder points. One of them might be faulty so sometimes there is a contact sometimes not. You could try to locate it by knocking gently with the plastic end of a screwdriver on the pcb. If vibrations cause the noise you could check all solder points optical or just resolder them. There are not too many. Also check all potentiometers! As they are mechanical parts vibration caused issues can be found here very often especially on their solder points.

Simple: connect the headphones out (3.5mm jack) to the input jack of your fender (6.3 jack). By doing this you will loose one channel so you better generate a mono signal inside your computer before. Electrically this is no problem, nothing will be damaged. Turn the gain and volume knobs down and apply a signal from your Mac. Gently increase gain and volume. Et voila wonderful mono sound through a guitar amp

the fuse hopefully took the brunt of the higher voltage,, replacing the fuse should do the trick, but yes you need the voltage transformer for sure they sell them pretty cheap here, Just be sure and look for the wattage in each one http://www.dvdoverseas.com/store/index.html?loadfile=catalog6_0.html

If you touch each wire, the one that receives the output of the tank should hum. Quick and dirty test.

Try here:[email protected].

Try this website:http://tubesandmore.com/ Look under reverb tanks. Check to see how many springs your old tank has and what size it is. I think Marshall used Accutronic tanks but have a look at all of them. There are pictures and info for each. Hope this helps.

probably a bad reverb tank. there is a little inductor inside that shorts out easily/common. you cant repair it. its just easier to order a new tank. what you do to test is........turn on the amp with the reverb turned up all the way........ volume up on the amp at normal playing level.....then tap or knock on the reverb tank itself......if you get sound then the output of the tank is good/ no sound the ouput side is dead.......then what you do is unplug the ouput of the tank and plug the output wire to the input of the tank and tap/knock on it again....if you get sound then the input side of the tank is good/ no sound is dead. this is a way to test the tank independently from input to output. which side is bad is irrelevant, what is improtant is determining if the tank is toast. i can get accutronics tanks from my distributor at a better price than anywhere else. email me thru my contact page on my website with the tank code numbers and letters so i can get you a price also check my price with whats on ebay, etc. my website is yostamplifier.com
good luck

Usualy bad solderr joints on the output pcb