There is a greater difference between the
the UA 2-610 and the Hardy or the Great River. The UA, while a fairly cool 'special effect' mic pre in my world is not the center piece for any recording facility. It has a very nice and musical sounding distortion, which is nice to juxtapose some individual tracks against the rest of a song but will lead you to a kinda washed out, indistinct, ball of snot if you try to use it over the course of an entire performance or project.
The John Hardy is a very much at the other end of the spectrum from the UA in that it is a very clear, detailed, yet opulent while focused sounding pre. The main differences between the M-1 and M-2 is that the M-2 has "stepped input attenuation" [meaning click stop gain increases rather than continuously variable... this would only be important if you will need to reset your mic-pre with like 1/10th of a db kind of accuracy], and the choice between an input impedance switch or a 20db pad. Frankly, the input impedance switch isn't really going to net you [at least in my mind] enough of a difference to warrant the price differential, and the M-1, by virtue of it's "High Gain" switch effectively has the pad option of the M-2.
The Great River MP-2NV... lemme start out by saying that I am exceptionally biased here because the unit was pretty much [OK, entirely] built to suit my tastes in terms of tone and function. Tone; well we went for something that would be clean, clear, yet opulent and euphonic but could be gain scaled to get a little nasty and aggresive should that be what the user desired. The amplifier is set up so that if you drive the input kinda hard, and back off on the output, you'll get a very sweet, musical set of distortions that when set up correctly will add a bit of urgency and immediacy to a performance.
On the feature front, there is an impedance switch that ranges between 300 and 1200 ohms [like was on an original Neve module... afterall, the MP-2NV did start with a set of Neve drawings... but took it so much further from that point that it can not be considered one of the "Neve clones"]. The input and output transformers on the MP-2NV come without their proper loading, which adds a ring to the transformers way up there, out of the range of human hearing, at 54kHz... however, the effect to the audio is palpable within the range of human hearing. With the loading switch disengaged there is high end "air" that is a by-product of this transformer ringing. The "loading" switch adds the proper loading to the transformers which nets a slightly different character to the audio... not better, not worse, but indeed different.
The MP-2NV also has an insert point between the input and output amplifiers... this can enable the user a wide variety of options... from taking the signal direct out from the insert point thus by-passing the output amplifier [a little cleaner path, fewer electronics always equals "cleaner"], it will allow you to pop an equalizer, compressor, etc. or both between the input and output amplifiers, but most importantly, it will allow you a remarkably wide variety of amplifier selections when coupled with the Great River EQ-2NV.
Like I said, I'm biased. That's what happens when you spend a year and a half of your life working on a project... you tend to be excited about the product when you come to the end [because if I wasn't excited, it would never have been released!!]. There are a whole bunch of other projects on which I have consulted, these, like the MP-2NV are called "Mercenary Edition"s...
Understand that neither I nor Mercenary Audio gets a design consultation fee, nor a royalty on units sold. The sole reason for "Mercenary Edition" equipment is so that I, as a recording engineer, have tools at my disposal that will net me a wide range of tones and textures, a set of functions that is unavailble from any other equipment on the market, and as much as possible saves on rack space. While the company I work for, Mercenary Audio, is a dealer for these products, we are not the only dealers for these products... so if you have a favorite dealer, we highly suggest you purchase these units from your favorite dealer.
So... as this is not a commercially oriented/spam kind of thing... here is a site where Mercenary Editions are explained in greater detail. Each and every product will give you a link to the dealers for that product, or to the distributors for that product.
http://mercenaryeditions.com
I hope this has been of some assistance without seeming like a sales pitch... FWIW, I own a 4 channel Hardy which I use all the time, love, and wouldn't want to be without. I think it's an absolutely wonderful and indispensible tool. The only reason I was foaming as much about the Great River pre as I did was that I am very proud of the work that went into it [Dan Kennedy did a phenomenal job!!!!], and happen to know a lot about it.
I hope this is of some assistance...
Peace.
) The deciding factor may be in the units' flexibility. I would imagine that the JH is pretty much plug-and-play. I mean, not much chance to change the pre's characteristics (by adding color, etc.) Does the GR have more flexibility in that area?