These are nice amps for servicing and quite reliable.
I'll start from the other angle and ask whether this is practical.  You will need a cab that is rated for 400watts at double the impedance of what the minimum is for each channel.  I'm pretty sure the minimum is 4ohms per channel.  So you need an 8ohm cab rated for 400watts or better.
  Now what's the chance it's a single speaker in that cab?  If it isn't, the easiest thing is to rewired the cab into 2 banks of speakers with 2 inputs.  Now you can get full power into a single cab.
  Otherwise, it's bi-amp capable, but also has a switch for mono, so the 2 amps are the same as Enzo mentioned.
  No big deal to make an adapter cable for the outputs, but you still need an inversion of one of the input signals.  This would probably be possible by reconfiguring the biamp mode.

  You will have a hard time finding another jack that will fit properly in there.
The part # you need is 099-0912-000.  It should be available online or through anyone that sells Fender parts.
  While you have it open, resolder the main filter caps.  Then see if any of the faults have been cured.
Service manual attached:

Quote from: Roly on April 23, 2015, 07:24:28 AM
  Never once have I seen a muffin fan with a brushed DC motor

Gotcha, thanks!  I probably quit taking fans apart around the time the DC versions became popular.  Up til then they were often expensive enough to warrant a repair attempt.
  So I had assumed they were brushed type but now I know better. 

I'm a little confused about a couple things so will ask for clarification:

Quote from: Hawk on April 22, 2015, 08:48:06 AM
Turns out the manufacturer has decided that the fan is not needed and the heatsink will be enough....guess that stuff happens all the time...

I thought this was about a circuit that you had in front of you as you mentioned a heat sensor and showed a picture of an actual fan.  Are you now speaking of a manufacturer of some other unit?

Quote from: Roly on April 22, 2015, 12:32:05 PM
The key thing here is to understand that the fan contains a tiny computer, a very simple one, but that still needs a certain amount of voltage to bootup and operate correctly, then the motor coil drivers are activated.

Roly, I had assumed this to be a very simple fan, but you speak of something more sophisticated.  Did you deduce this based on the photo or the resistance measurement?

  Now the ext.spkr. jack should be able to power a speaker with no further amplification necessary.
  Incidentally, it is hazardous to plug a powered out like that into a regular input.  Normally there is enough level there to do some damage if you're not real careful.   

  Some of the 18V will be dropped across R108, some across Q102.
There should be some measureable voltage at the transistors.  Check that R108 is getting it's 18V.
  We do not have the amp so there is no way to say what the component used for RT101 physically looks like.
 

Quote from: DrGonz78 on April 03, 2015, 05:10:34 PM
Are these class X and Y the types of caps in this Roadmaster amp? I have come across these caps before and had only a vague understanding of their functions.

No, these are DVL type caps (double layer?).  That was what was spec'd prior to classx&y for safety purposes (after it was realized caps used as "death caps" needed to be special). 
If this amp were being built today, they would have to use a classX for C59 position, and a classY for C60 position.  C60 is what would be considered the traditional "death cap" position.

Quote from: Roly on April 04, 2015, 08:19:54 AM
But the multiplier runs from the nominal 60VAC coming out of your transformer secondary (a transformer you already have?).

If you have the existing transformer, just use it as JMF mentioned.  It has a 200VAC winding for the tube HV, as well as a heater winding.

Schematic attached:

schematic attached:

It is best not to use the terms class X and type Y when referring to positions or functions.
What classX and classY denote is that they are specific caps approved for those positions.
ClassX is for line to line, classY is for line to ground. (neutral here is considered line, chassis is ground)
For example, a cap going from line to ground (what we call "death cap") is not class Y unless we specifically buy an approved "class Y" cap to use in that position.

http://www.justradios.com/safetytips.html

  I doubt that having those 2 pots ganged together will track the way you expect.
Also, you are vastly limiting the tonal possibilities you would have if you use 2 separate pots.

Quote from: Enzo on April 01, 2015, 12:11:02 AM
And we charge very reasonable rates...

You'd be worth it even at ten times your rate  .

Here's the user manual

  JM outlined the advantage of this design above.
The downside is that you need separate power supplies for multi-channel amps (as shown by the separate transformer winding in the QSC example).

  There is a version of the LM386 (N-4) that is good to 18V.