Is that bias voltage on Pin 5?  Those should be negative numbers.   But they will be meaningless if you cannot read the plate current on Pin 8.   You need to find out, from some of the other guys who have built this amp, what kind of voltage they have on their screens.  Yours seem very low to me, but I have never been in one of these amps.

I really have not looked at any of your pictures.  But from your chart it seems like you have B+ problems with your screens, they should be much closer to your plate readings, and you have bias problems.  Your bias voltage should not have that disparity, the two readings should be very close to equal. Also, you need to have a meter that will read milli amps between the cathodes and ground.

Hello -
Other than the one EF86, isn't this a standard 18 watt chassis?   I assume it follows the Marshall schematic pretty closely.

http://www.18watt.com/storage/18watt%20Schematic%2008-28-03%20-3.pdf

http://www.18watt.com/storage/18w%20Schematic.gif

Maybe I do not understand your question.  Why do you mention Vox and Matchless?   
I have never had my hands in a 18 watt Marshall.  I apologize if your query went over my head.  The guys at 18watt.com should be able to help you.
Good Luck

I assume V4 and V5 are your Power Tubes.
Pin 3 seems normal.
Pin 4 seems pretty low. Should be closer to 450.
Pin 5 should be -VDC.  Something like -45VDC.
Pin 8 should not be Zero.  Should be milli volts.  Perhaps something like .035 VDC.
Does that make sense to you?

Pin 3 is plate voltage.
Pin 4 is screen voltage.
Pin 5 is bias voltage.
Pin 8 is your plate current across what I assume are 1 ohm resistors.
Are these 6L6 power tubes?

So your amp has an impedance selector switch?   From looking at the schem on this website it did not show one.   Regardless, the jacks are wired parallel.  If you plug a 16 ohm load into each jack, you need to select 8 ohms.
Good Luck

I would like to see a few better pictures of your amp, if you get a chance.

Just two completely different amps.   If you have (and like) a '59 Bassman, I think you would have more versatility with the 18 Watt.  Just from the perspective of having two very different amps that also have a pretty big disparity in volume.   As you said, the JTM-45 (or your Bassman) will be too loud for some occasions.   It would be nice to have a lower watt amp to cover yourself when you encounter a "volume sensitive" situation.
Good Luck

What is so good about the Fulltone is that it seems to work real well with a lot of different amps.   I would also mention:
1.  T-Rex
2.  One of the many modified versions of the Boss BD-2

Wow!!!
Congratulations!!!
You did a good job.   I really admire the way you stayed with the amp through every problem.   It's easy to get frustrated and give up with some of the problems you had.   Thanks for taking me along for the ride.  I learned a lot.   I look forward to your video.
Well done, and have fun!!!

That is great news!!!
So you had 2 bad tubes... Oh Well.
The 4 ohm speaker will not effect your bias like that.   I would say if your bulb looks OK you can disconnect the limiter and check your bias again.   If you can not get past 4.6 MA I would change the value of the resistor next to the bias pot  (is it 52k ?).  You can even lift one leg of that resistor and use jumper wires to try different values.   When you get to a resistor that seems good, take out the old resistor and install the new one of correct value.  That would be the sum of the two resistors that you had jumperd together.

The 1987x schematic should tell you all you need to know for the burned up parts.   I would contact Dagnall for a wiring diagram for the transformer.
Good Luck

OK.
Those Valve Arts are supposed to be the ones to get as far as modern day tubes.   There aren't many options, outside of NOS KT66.
Keep us up to date.
Good Luck

By any chance, are your tubes made by TungSol and do they have a metal base?

Wow!!!
Very nice.   Well Done.
Where is the red wire going to from the speaker jack?
And I am trusting you to be honest with me on this next one..........Do you hear a difference if you change the phase while your playing?   I have read many threads about the "polarity" switch.   Strong opinions on both sides.
Thank You

OK, now you have lost me.
It does start to seem like your tubes are shorted inside.    Both tubes though..... it happens, or they could have been damaged in shipping.    I guess  you will have to order some new tubes.
It would seem that you already proved, with the 50 ohm resistor, that there is normal current flow between Pin 2 & 7.   But once the tube goes in, that heater voltage seems to be getting shorted in the tube.
Can you beg, borrow, or steal a tube from somebody?   It can even be a 6L6, it does not have to be a KT66.   Just as long as you know it is a good tube, you can put it in and see if the heater voltage stays at 6.0 VAC.