Does anyone know if this mod sounds legitimate on my Jcm 800 4210 1x12 50watt marshall combo

Locate the 10K cathode resistor (R13 found on V2) and place a .68uf cap across it. This will increase the gain Alot!. It also boosts the mids nicely to get that sweet Marshall "crunch". If you're into classic rock, you may not want this cap on there. If you like "heavier-tones" then run with it.

thanks guys
rock on
No, this seems to be a mod for the 2205 head series.
It's not one that I would recommend anyway.
It adds an aweful lot of gain in one stage which can sound brittle and noisy.
It's best to add little bits at multiple stages.
There doesn't appear to be that many places to add additional gain in the 4210, there should be a lot of gain on the boost channel already, looking at the schematic.
You could try increasing the feedback resistor value, from 100k to 220k perhaps, to decrease the negative feedback and increase gain or try changing two of the silicon, IN4007 diodes in the clipping circuit to germanium type. That will increase gain and create asymmetry in the clipping which sounds warmer.
the last mod you said sounds appealing. something i could do myself or should i leave it to a professional? i have basic soldering skills and a real understanding of boards. i also feel like i can get the clean channel on my amp much louder than the overdrive channel, anything i could do to cure that?
On the diode clipping: if you're willing to take the risk and do it yourself then I would say go ahead, it's not a massively complex procedure.

On the the levels of the two channels I would suggest the following: Could you replace V1 and V2 with known good valves. V2 is particularly important to the boost channel. The reason I say this is because the boost channel has 3 gain stages, V1b, V2a and V2b. Even though the signal is heavily clipped, it should produce reasonably even level between the clean and overdrive channels.
There are a number of points where you could either attenuate the clean level or boost the overdrive slightly but I'd prefer you to check if you have an aging valve causing the problem first.
It's always best to fix a problem and not a symptom!!
thanks man, ya i just had all the tubes replaced and a full tune up of the amp so everything should be working well. im just lookin to push the drive channel a bit more because i love what i get out of my clean channel when cranked but the drive channel sometimes just feels like its lacking a bit