Alright, so there’s a lot of information out there about this, I’ll try and sum it up quickly.

If you have a 100w tube head, it’s probably too loud, right? As in, you turn it up past 4 and it’s too loud? If so, then you probably are going to want to get an attenuator, like a THD Hot Plate or a Weber Mass. That helps a little bit - it lets you turn your amp up louder, which hits the tubes harder, which gives you that sweet cranked tube mojo sound we all love. Another option, though, is to use THD Yellowjackets. They aren’t cheap, and you need to make sure they’ll fit in your amp, but basically they’re low-wattage power tubes that you plug in to your high-wattage power tube sockets. This lowers the wattage of your amp by 20% or so, so not a whole bunch, but it lowers the headroom - you get that cranked sound at lower volumes, in other words! So if you’ve ever turned your amp up to 7 and been like “dang that sounds good, too bad I can’t hear the drummer” you might want to look in to one or both of those options - it’s nice to plug in and get that sound at 3 or 4 instead of 7, y’know?

So…. power tubes. You want a matched set of power tubes, so you’re going to want a quad. Don’t mix types of power tubes, don’t mix brands, the safest thing to do is buy them as a matched quad, that way they’ll all draw the same current and won’t be unbalanced, which means they’ll all last longer. So quad = quartet. Make sure it says “matched”.

Me personally, I’ve only tried the ElectroHarmonix EL34’s, and I really liked them. I put them in my Mesa Boogie Tremoverb, and was pleasantly surprised to hear a very “Marshall” tone come out… probably the most significant tonal change I ever did to the amp! Compared to the stock 6L6’s and the Mesa 6L6’s I eventually put back in there, they had tons of sparkle and a great snarling midrange, although much less of the lowest bass chunk that Mesa’s are known for.

It’s hard to know much from the rather limited description that Musiciansfriend.com has on its website. I managed to find a brief discussion on another forum, link below, third link.

In a very general sense, EL34’s are midrangey and trebly, 6L6’s are more even (not as chiming or detailed in the high end) with more bass, and KT-66’s are somewhere in between. Stick to the EL34’s, I think, for the “Marshall” sound…. just make sure that your amp is properly biased for it. If you don’t know what I mean, talk to a tech about it - it’s not a simple topic, and I don’t want you misled.

For preamp tubes you don’t need rebiasing. You will want 12ax7’s in your preamp, except for your phase inverter, which will probably be a 12at7, if I remember correctly. That stays the same. In terms of gain 12ax7 is the highest, with 5751’s at about 70% of that level. You could put 12au7’s or 12at7’s in there, but they’re much lower gain… less than half the gain of the 12ax7’s. The less gain you have in your preamp, a) the less distortion you’ll have and b) the quieter you’re going to be, since there won’t be enough gain to make up for signal loss in the tone stack.

So… I would suggest sticking with 12ax7’s, unless you specifically want a cleaner, less overdriven sound.

As far as preamp tubes go, I love JJ’s - while some have criticized JJ’s as being less reliable than other brands (like Sovtek, for instance), to me the plain fact is that they just sound better. I gigged with my Mesa for a year and a half, and never had a tube fail on me, and I wasn’t totally gentle with it, either - most of the time I didn’t carry any extra tubes even (naughty me). I would specifically recommend the JJ ECC-803S’s, they have a spiral filament winding that reduces noise, and longer plates for better low end response (aka the chunk factor!).

Some have criticized the JJ’s for being “dark”, but I think that in high-gain situations they’re excellent. Too much high end in a high-gain situation isn’t good times, after all!

I saw a kit on musiciansfriend by Groove Tubes that was specifically geared towards Marshall, I’ve added that link down there too.

Personally, I feel that I could probably get more bang for my buck by buying an assortment of different tubes and just trying them all in turn, since you can buy most tubes for less than 20$ at your local music shop or online retailer. I highly, highly recommend focusing on a high-gain spiral filament wound tube for V1, the first tube that is - it will make the biggest impact as far as gain and lower noise goes. V2 can be more of a “color” tube, it doesn’t strictly have to be the spiral filament, I would feel comfortable experimenting with that one. For me personally, I can’t *stand* noise, so I put the JJ’s in my V1 - V3 (the Mesa has three preamp tubes, your JCM has two, I think), and an ElectroHarmonix 12AT7 in the phase inverter.

Feel free to email me if you want, I’m happy to offer my opinion or answer any questions for you.

Cheers!

EDIT: I’m very sorry, Y!A went belly-up on me and destroyed my links. I apologize! You may have to browse, google, or contact me for more….

Saul