Im surprised at so many of you saying you'd prefer the M-Field. I came to the other conclusion but to each their own. I'll certainly read the reviews and comparisons.
I chose this route because I want a more "tubey" sound, as I strongly prefer it, and have found the Tungsten's sound amazing with tubes. 300B amps are way out of budget, and while the HA-3A more than adequately drive the Tungsten's, the class A nuclear power of the Mentor/M-Field has me too curious to pass up whether or not there's still more to gain.
To echo what's been said about the two models already, here's part of the email I received from Flux a week or two ago regarding the two:
"...Regarding the sound features. Let's try to describe the difference between these two models briefly. We made great efforts to not lose detail in the M-field model compared to the Mentor. However, it stands to reason that the Mentor will be the most detailed amplifier with the largest soundstage of all. At the same time, the Mentor's bass is not as weighty as that of the M-field, but it has a more complex structure, which is unattainable by most amplifiers on the market. On the other hand, the M-field has a stunningly soulful and intimate midrange that only the best tube headphone amplifiers can deliver. At the same time, the bass of the M-field is delivered a little softer and more weighty.
As a result, we got two completely different sound settings. Mentor has the most uncolored, accurate, fast, and detailed sound. And M-field is a more intimate, enveloping, weighty, and warm sound.
In terms of wattage, these are comparable amps. Both models allow you to create a stack of two devices."
I'm sure I'd be more than satisfied with either version of the Flux, even the single units, but it'd always leave me wondering what if.