I like the 02 caps and if you use both the Strat and
Humbuckers.. you may want to keep the bypass cap across the
brite channel.You can try the shared or split cathode to see which
you like.If you use different guitars.. it would be good to set the one
channel for the strat and the other for the humbucker type guitars.
You can change the feedback resistor to keep it cleaner.. maybe
around 33k..
or even a switch to disconnect the feedback[for a little boost}.The
main thing is get good preamp tubes.. so much tone, or how the
amp breathes will be because of what tubes is in these amps. I
would use the EL34 OT tubes.The early Marshalls were copied
mostly from the bassman 5F6A..so i think thats why the early ones
used the more 6L6 type KT66 or bigger bottle type tubes.. But i
think of a Marshalls being an English amp sound. and that is EL34
tubes. You could change the resistors to give more voltage to the
power tubes and less to the preamp tubes. I agree with the plexi as
being a great amp..the .68 is a good choice and even the mid pot
change works pretty good too. gives a lot more addition to the
amp. I like the bass amps..And i like the solid state rectifier.{just
me} Set the amp loud and clean as you can and use the TS for the
extra kick. This is hard to say for each amp may have its own
signature tone.. then you have to work with what you have.
Richie{~}==::: 

Replace the 50X50uF
caps with a 32X32uF adjacent to the GZ34, and a 16X16uF at the
other end of the chassis. 

've actually done the whole conversion now. It took a lot longer than
I expected. The tone stack of a '78 1986 is totally different to the
earlier models. Had a 0.047 treble cap, 100k slope resistor, 1 meg
treble, 1 meg bass and 220k middle pot. Quite bizarre.

I've used the '70 schematic for the 1987 fairly faithfully. If I have to
do any further mods I'll consider using a Hoffman board for it. The
PCB is really difficult to work with.

The problem with the reissues is mostly tubes, filtering, and cold bias. Grab a good used one and do this :
JTM45 Reissue: Valve Art KT66, Ei ecc83, 16u x 16u preamp, 32u x 32u poweramp, rebias.
Plexi50 Reissue: Svetlana EL34, Ei ecc83, (3) 32u x 32u, rebias.  

1. Part of the mid-heaviness is the OT; it's normal for 70's amps.
Adding more -fb will help ease that.
2. For more highs and lows, go to the 1986 tone stack (250pF, .02,
.02 and 56K slope) The tone controls will do "more". 
3. The overly bright Ch-I and bassy Ch-II are what leads most folks
to jumper the channels - so you can blend them to suit your
instrument and playing style.