If you are familiar with the Clubman, in what ways does it differ from other EL-34 amps such as the JTM50 or the TW Expression?
Do EL34 amps in general sound a lot alike?
No. Not at all. The power amp tube does have an affect on tone and, especially, break-up characteristic, but the preamp tone stack design is the voice of the amp. Vox made amps that ran with E;84, El34, and even SS poweramps that all sounded like Voxes...why? They all had similar preamps.
Would there a be a huge difference in the clubman and the JTM50 for example when the only obvious difference is the preamp tube.
Huge difference. Obvious difference is misconception. It's not just the preamp tubes, its the cap values and placement of the tone stack, as well as cathode followers, limitations on gain increases from stage to stage, etc. There are dozens of choices in preamp design that has an affect on tone and distortion.
What does a rectifier do, and do I need it?
All amps have a rectifier, it converts the incoming AC current to DC current. in most amp designs, use of a tube rectifier adds sag, the attack of the note is muffled and then blooms. Sag is popular in blues, the faster attack of a SS rectifier is more popular for rock (especially harder stuff).
Would it be feasable to modify one of the existing ceriatone kits to make a clubanish amp? (i.e. Convert the DC-30 to an EL34 Output stage or change the preamp on the JTM50 to a pentode.)
Yes, the DC-30 has a similar preamp to the Clubman, so it's a matter of building and attaching the poweramp, the Matchless EL34 poweramp schematics are out there, but it depends on either Nik or another builder wants to tackle it.
Or...is the major difference between the JTM50 type and the clubman a Class A vs. Class AB thing?
Not really.
I also really like the sound of the Overtone Special. That is a 6L6 amp. I've seen things that suggest that it's easy to change EL34 amps to 6L6. Is this true?
Yes. As long as the power transformer is beefy enough and the tube sockets are wired right. The 6L6GC have a bigger, more open sounding tonal range, the EL34's have more midrange.