I think you mean Dana Hall.....
Isn't it just two wires depending on Cat or Fixed bias.  I am sure Hall will give instruction if you buy one of their devices.
Good Luck

To me it looks like both rhythm and lead are switchable.
http://www.ampwares.com/schematics/marshall/25aniv_silverjubilee_50_100w_2555.pdf

Congratulations!!!!
You really "hung in there" with this amp.  Tubes are one of the first things to suspect when you have a problem. They can, and do, go bad at any time.  They can be fine at the dealer, and go bad during shipping.  They can be perfect at rehearsal, and go gad at the gig.  They can work fine while you trouble shoot three other problems, and then die themselves just when you think you have it all fixed.  If you have a tube amp, you must have spare tubes. It is that simple.
Good Luck

Nik does not normally supply a schematic. I imagine his reasoning would be that his amps are either a matter of public record, or they are proprietary; and not available at all.  Your amp circuit is pretty much communal at this point.
Good Luck

http://www.ampwares.com/schematics/marshall/jcm800_lead_mstvol_100w_2203.pdf

The HT fuse is usually power tubes, but in a new build it could be a number of things.  I would check all of your wiring against a schematic, not the layout.  Check every connection on the schematic with a bright marker as you verify it.  It is also a good idea to run through the chassis with your meter and check continuity. Make sure nothing is shorted to ground, if your meter talks, all the better.
I would do those two things; check continuity, and verify your connections to the schematic.
If you have or can borrow a set of power tubes that is another quick way to see if your tube(s) are bad.

You must still be pulling to ground somewhere. What were the plate voltage and current when you set the bias?

Try switching your OT leads.  I assume yours are Brown and Blue.  Those feed one set of power tubes to the OT.  Just swap them, Blue to where Brown is,  and Brown to where Blue is.

30 milli amps would certainly be a common ground.  But you need to consider two other factors:
1.  Your Tone.  How do you like the way your amp sounds and responds to your touch with a certain bias.
2.  Milli amps by themselves are only half the equation. You need to check you plate voltage and make sure you are not running too much past 70%-80% dissipation.  At some point there are diminishing returns regarding tone and tube life.  Certainly if you are red plating your bias is too positive.
Good Luck

Basically you put a light bulb in series with the hot lead of a cord that attach's to another outlet. Plug your amp into that outlet so your wall supply is in series with the light bulb. If your amp has a short, the bulb will be bright, if the amp is good the bulb will be dim. I can send you a picture of mine along with a schematic link.

I am sure the fuse was good. I am starting to think you have a short somewhere in your B+ wiring.  Maybe in the rectifier diodes or maybe in the filter caps.
A light bulb limiter would help, saves on fuses, and is pretty easy/cheap to build.

The two wires from the PT to the SB switch. That will tell you if the high voltage AC from the transformer is conducting. If that is OK, you can follow it across the SB switch to the rectifier diodes.  It is possible you have a bad SB switch. I just replaced a bad one that was grounding the B+.... so you never know.
Do you have your diodes oriented the right way?

I am not sure what point K is, I assume it is right after the fuse where the choke and the OT meet.  If you turn the standby switch off (open) what do you read at the two HT wires?  They will be/should be high voltage AC, before it hits the solid state rectifier.
I am always a little confused by Zero ohms. Is that the same as continuity, is that what a new fuse would measure with your meter?  Just wanting to make sure your fuse is OK

Is your HT fuse good?  Otherwise you need to just backtrack from the screens ( Pin 4 ) and their resistors, across the choke, filter caps, rectifier, stand by switch, to the transformer.  It seems like you do not have any B+ supply, it must disappear somewhere.  Do your screens have DC voltage (Pin 4 to ground) ?
Is this EL34 or 6550 power tubes ?
Do you have a light bulb current limiter?

Just to double check.... you are reading the negative VDC from Pin 5 to ground, and your 1 ohm resistor is attached from Pin 8 to ground.
Is your meter auto ranging?  If not, you need to select the lowest possible range for VDC.  This will allow you to read milli volts DC across the 1 ohm resistor.

When you say 42.5, you do mean negative don't you? Is your bias cap positive ground?  I think on those JCM's it was a 10uf/150 volt or somewhere in that range?