MKB

Hard to describe in words. They definately have their own sound.

Clean channel (R1): Very clean tones, and can get very bright. The clean tones are excellent, and it works well with pedals.

Crunch channel (R2): Almost useless. This mode sounds like a bad distortion pedal, especially in the bass. I have tried quite a bit to tweak this channel with no success. There's just not a good tone there.

Lead channel: This one sounds, well, Marshally. The channel has two gain knobs, one is voiced trebley and the other is more mids and bass, you can mix the two for different sounds. You can get some very good sounds out of this channel. It has more of a modern JCM900 sound to me, not so vintage sounding.

The preamp has an interesting circuit layout, mostly op amps, with three 12AX7 tubes running off high voltage. All of the channels have at least one tube amplification stage. The tube arrangement is unusual for Marshall, they use a topology called a mu-follower (where a tube stage replaces the plate resistor) for some reason. This makes the distortion tones sound slightly softer than you might expect. But overall the lead channel sounds much like a Marshall in tone. FWIW, the preamp uses diodes in all distortion modes. The effects loop and five band EQ work very well, and are footswitchable.

I made some modifications to mine, basically changing the distortion diodes to mosfets, and changing the mu-follower arrangement of the tubes to the standard cathode follower (like most other Marshalls). This actually helped the tone quite a bit, especially in the lead channel. This modification gives it almost too much solid bass in the low end, and not muddy bass either. Overall I really like it, but it is seeing a lot of closet time since I got my Vintage Modern...