First off my thanks to deltafred for catching my dropped decimal place. I was possibly overzealous in impressing OP with how weak an instrument is for driving a power section.
Before I address this new data let me speak on measuring signal voltages. Unless otherwise noted in your schematic, all voltages are measured relative to ground, also called "common" for exactly that reason. It is highly unlikely that your meter is protected against DC voltages while in AC mode and more to the point there is no need to risk damaging your meter by attempting to read signal voltage from a location carrying DC voltage. Just take your measurements AFTER the coupling capacitor in each stage. Inputs are low voltage and generally safe for your meter but possibly bad for your amp - note the squeal you heard. The easy way to avoid all this is to attach a 600v .1 mfd cap to your positive probe just to be safe.
Now with reaching 0.84vac at amp input you will probably be creating some overdrive distortion but at least it appears you are getting decent gain in the preamp. Obviously with your Master Volume at 1 you are going to have an extremely weak signal at the power section. If the amp does not presently have a Power Amp In and if you don't wish to add one permanently you can easily do so temporarily for the purpose of testing.
Here's how I would do that. Firstly, referencing the output schematic in your very first post, I would solder the hot lead of a phone jack to the .022 cap on the side opposite what is referred to as V5-A. There is some DC bias voltage on the grids of the inverter/driver stage and not knowing the impedance of your meter (one of the things that separates hi from lo quality in meters) it is best to not take the risk of altering that bias voltage which is why we choose to make the jack connect opposite the tube grid. As for the existing connection it would be best to lift that out of the circuit but in most cases it will work regardless. You may note in schematics for amps that come with such jacks, that both interrupting and parallel connections are chosen depending on desired operations so you just can't go wrong here. It will work so that you can input your 0.84 vac signal directly to the power stage. It will not drive the amp to full power since roughly 2.0 vac is required for that but it WILL BE LOUD assuming the power stage is working. Because of this have some means to vary or turn off that signal or utilize the standby switch so the loud is brief.
Once verifying we have LOUD at the speaker at less than half required signal in at the driver stage, then I would proceed backwards and solder the hot pin of the jack to the high side of the Master Volume. By varying the Master you should be able to get from no sound to the level you got in the direct test above. See? Process of elimination working back from output to input. If this works, at this point you could reconnect the FX Loops and observe the results. I'm betting you will find the culprit, or at least the location in which to troubleshoot specifics, at this point.
It would be instructive to repeat these tests for max output once you manage to get 2vac at the driver input. A simple boost pedal like the "divided by 13 Lift" pedal
http://www.dividedby13.com/accessories.html would do a proper job but there are cheaper alternatives and once we have verified even near full output and have discovered where the loss is, we can do this more normally in a musicians pov by just plugging in your instrument since the preamp was originally designed to produce well over 2vac at the driver.
PS Almost forgot. Did you try advancing the Master beyond "1" to see if the amp got loud with the loops disengaged?
(ran out of it)
