Building a pedal to add thickness/beef up to my tone

[runmikeyrun]

funny, i was just posting about this on the 800 topic, try lifting the clipping diodes in the preamp and see what that does... schematics for your amp are available with a google search and they shouldn't be hard to find in the preamp.

if you've never worked on a tube amp make sure you know how to discharge your amp before you start poking around with a soldering iron because you can be killed.

[soggybag]

I think you need a ring modulator. This is will create the flavor of a rancid steak tartare. Raw in texture with many subtle flavors, completely unhealthy.

[brett]

I think you need a ring modulator.

Now that was completely random!
I agree with the fat strings comment, but with the wrong settings that 900 will always turn wine into vinegar.
Replacing the first 12AX7 in the chain with a 12AT7 or a 5750 (or just an old, weak 12AX7) will help a lot. 
In my experience, many NOS 12AX7s from e-bay are feeble.  IMO many people like them coz they are in the same situation as you. An test (emission, or preferably transconductance) about 70% of new would be good.
cheers

[col]

I have had to play through a lot of JCM900 series amps as we usually have to use whatever backline is at the gig. They are all so different to each other they could be different amps from different manufacturers. If you get a good one they are great but I've played through some that are exactly as you describe with an anaemic tone and no power whatever you set the amp to, and that's with an SG. I went to buy one a few years ago after I'd had a very good experience with one but ended up buying a second hand Laney AOR50 as it was the best sounding amp in the shop and easily as loud as the JCM900 100 watters I tried at the time. Shortly after I got a AOR100 head cheaply and these have covered all my needs since. The 100w version is simply too loud though and I use the 50w one 99% of the time. I hve never had a problem with weak sound while using these amps, or on the occasions when I've had to use someone elses Laney.

[Processaurus]

Some Marshalls are funny, I know a guy that plays one (sorry, don't know the model) and has this infamously insane, piercing thin tone.  I played it after him at a show because I was too lazy to bring an amp, and I expected to walk up and see a wack setting on the EQ, but no, everything was straight up 12 o'clock!  It hopelessly messed up with my guitar too.  To get a normal sound like you would get out of any number of amps with the tone knobs set dead center I had to turn the treble AND mids all the way down to zero, bass almost all the way up!  Once the treble and mids were down it was like the bass knob brought in the normal, flat sound (including a nice amount of treble and mids).  Strange interaction between the controls.  Weird amp...

[GibsonGM]

It's also a tube characteristic to be a little harsh on max gain, I find.  Add to that a 'naturally shrill' guitar, and that can sound crappy.   I back my gain off to about 6 or 7, and use the LPB to fatten my tone up if I'm playing a Fender.  +1 for getting the other stuff solid first, tho (pickups, etc...switching from 12AX7 to 12AT7 in V1 helped my tone, too).   I use a Fender amp, so go with the Marshall guys' suggestions first, LOL.   The LPB is for heavier material; my amp is made for blues, and that's what it does best.