Now we deserve to see the winner's works.

Hi soundroi.
Whatever the User Manual says , the schematic shows clearly that the "signal output" is the speaker output attenuated by two resistors: an unspecified one but around 22K, and a 2k2 to ground; that signal goes to the "Signal Out" jack, just look a little above the speaker outs.
It won't work if you unhook the power amp.
Above the power amp itself you'll see the RG100HT, which must have been an upscale model, sporting 3 outputs: a jack one, an RCA one, and a low impedance balanced one.
All three come from the speaker side and all said before applies here.
Now that I lok at that scematic: the solution to your problem is
1) your real Pre-signal out is the effects Send jack.
2) You *must* use a shorted plug into the Effects Return to kill all signal that might reach the power amp.
3) "Tame" your Randall by soldering a 15K resistor from the left pin of R45 to the right one of R46.
Left and right as seen on the schematic, on the actual board they might point anywhere.

The little Gem will be easier and with a regular pot (5K to 100K, preferably audio taper) on its output, can work as a preamp and even provide distortion, good idea.

If you tear the amp apart, probably all you will be able to use is the power transformer, if anything.
If the amp still works, check that it can drive 4 ohm loads per channel, then you can hook both hi fi cabinets to one channel, the 4 ohm 10" subwoofer to the other, that should provide a reasonable volume for home practice or light garage rehearsal duty.
You could build a simple 1 transistor preamp to pick up your bass signal and send it to both line in (RCA?) jacks.

Hi gianni.
The answer may be easy, may be difficult, maybe anywhere between those two extremes.
Could be flying among the clouds on a clear summer day, or hidden in the deepest trenches of the Pacific.
Who knows?
Fact is, it can go on unanswered forever, unless you tell us where do you live, what's the power voltage there, and what your amp states on its back panel.

Hi alex.
You did not mention the single most suspect connector: the loop in or power amp in jack.
Clean it too and even push the leaf closed with a piece of wood or plastic.
Even better, just replace it.
To test it , hook your guitar into a distortion pedal, clean preamp, multi effects , or use another amplifier's pre/line out or a CD/MP3 player, or similar and hook its output into the power amp in.
Post your results and good luck.

Then, as Enzo guessed correctly, your reverb sprongs are feeding back because the speaker makes them vibrate.
Manufacturers use various "medicines" to avoid it, none of them very effective, but if you use many ....
Obviously that defect didn't exist when the amp was new.
Manufacturers do:
1) the spring sub-chassis "floats" inside a larger metal box, suspended by 4 tiny springs. If one or more stretch or unfasten and the subchassis touches the box: it feedbacks.
2) that box is held to the cabinet side or bottom by 4 screws that go through soft rubber rings, If they are old they crystallize or become a sticky goo; replace them.
The screws are *not* tightened, they are loose and let the box bove around a little.
A typical un-experienced owner tightens them. Mistake !!.
3) Fenders wrap the box in cotton-like padding, and stuff that into a Tolex envelope, lightly fastened to the bttom of the amp.
To avoid the cotton touching the reverb springs, they close the box with a piece of cardboard.
I see you get the point: "float" or decouple mechanicaly the reverb tank from the vibrating cabinet.
Some players, Paul Young and Dick Dale come to mind, mount their reverb tanks in another enclosure *away* from the main amp, even in another room or under the stage.