I have several guitars, mostly humbucker / locking tremolo incarnations, but this was also used with single coils and P90 (also for testing purposes) over some years.
We can discuss forever about the benefits or drawbacks of hybrid technology (that famous bridge rectifier BR3 diode!) but is a fact that transistors are not tubes (or valves) when it comes to guitar amplifiers, in particular if you want distortion out of them. What many manufacturers do (certainly including Marshall) is to present some amplifier (e.g. the Valvestate line of the brand) in which one of the amplifiers stages (commonly the power section) is based on tubes and the other link in the chain (the first one, actually, the preamplifier which shapes and define the tone of your amplifier) is based on tubes. Commonly, the preamplifier tubes are rudely more fundamental in producing your amplifiers tone, are easily replaceable, last longer and dissipate less heath, hence being this approach a logical one. Unfortunately, tone is not exactly a logical concept, and the lack of tubes in the power section certainly affects the overall tone and feel of an amplifier. All the foregoing is particularly true if you want your amplifier to produce some serious, rich and punchy distortion. Transistor amplifiers could be far more sterile, but they are theoretically more apt to produce clean tones than a valve amplifier.
What happened in this case is somewhat different, being a dedicated preamplifier. The fact here is that the unit has two tubes (one for clean tones and the other for overdriven, hot tones) and the amount of distortion that you can get out of one tube is technically limited by physical definition. To cope with this, and to produce the legendary raging Marshall distortion, the JMP1 relies on a diode (the famous BR3) that (in very pedestrian terms) excites the corresponding ECC83 tube, boosting its ability to produce distortion.
Pros? A lot. This is probably the best preamp to record with IMHO(probable exception made by the Damage Controls Demonizer), with nothing more than a pair of 1/4 cables, straight to the mixing board. No hiss or noise will bother your playing during the process. The tone is perfect to me, so I guess that Marshall did a good job here. Some people dont like it, but I dont know what theyre after.
Another department in where the JMP1 shines is flexibility. You can pull tones out of this baby that would require at least 3 or 4 conventional Marshall amplifiers (and you can switch between them seamless via MIDI!). Let me stress again something I consider very important, no matter how obvious should be to some of you out there. You wont get Soldano, Mesa Boogie, Matchless, Budda, Demeter, Carr, Fender or Vox tones with this (not even simulations or resemblances), anything coming out of the JMP1 will sound true Marshall, which is good news to me in its own right.
Cons? Very Few. One VERY annoying missing feature is the fact the only volume knob in the front of the unit (hardware potentiometer, that is) controls the output to the power amplifier outputs, rendering the control totally useless for the home studio applications. Of course you can still control the overall volume in your headphones and the direct recording outputs through software, but it would be very nice to be able to do so the traditional way. Continuous Controller capability and phantom power would also be nice to have in a professional piece of equipment like this.
Never broke in several years. Tubes replacement was certainly required after some 2 years or so, being replaced by Groove Tubes, Svetlana and Electro Harmonix Russians. Cleaner with the former and much muscular with the latter, the unit delivered the goods every time.
I own lots of gear and I'm involved in a home studio project. In consider this piece of equipment a valuable tool, mainly for recording applications, even while I feel shall perform equally well in live envornments. For testing purposes, I grabbed what I got (namely a Mesa TriAxis, a Damage Control Demonizer, Mesa V-Twin, Mesa Boogie DC3, Marshall JCM900, ADA MP1 and my lovely ADA MP2) and started some serious shredding sessions, comparing tonal outcome. For testing purposes, I hooked the machines to both my Marshall 4x12 cabinet and straight to my Behringer EURODESK FX PRO 2442. The results are my opinons above. I did not favoured this one, I kept them all.
The only thing I hate about this, as I said, is the fact the only volume knob in the front of the unit (hardware potentiometer, that is) controls the output to the power amplifier outputs ONLY, rendering the control totally useless for the home studio applications.