While I've spent more time living with an AI M2D than Carl has, I'm not sure I can say I have more experience with it.
My 2D has faithfully sat on my shelf for nearly 20 years (oh my, how can that possibly be?) dutifully linked to a B&K St202+ and my beloved B&W 802f specials. I have tinkered a bit with different groups of tubes, but have never so much as replaced a power cord or tweaked it one bit. (Carl has. See :
http://forum.audiogon.com/cgi-bin/frr.pl?rprea&1027541416&read&3&4& )
I love my preamp. It has never given my a bit of problem and has always sounded clear, rich, substantial and warm. Mostly it made music sound more like music than any preamp I'd used before. Shortly after my acquisition of the M2D, two events occurred: 1) the birth of my first child and 2) my departure from the quasi-high-end audio dealership where I worked. Parenthood and the lack of day-to-day with the toys managed to taper my interest, so I never really explored much beyond the system I've come to know and love.
Part of the fun of owning a tube preamp is the ability to try a variety of different tubes and to see how hey each affect the quality. Carl calls the 2D "the Tube Eater," which I can neither confirm or dispute. I have never owned another tube preamp, so I have nothing by which to compare it. Even so, if it consumes tubes the way a car crusher consumes automobiles, the fun is in the installation and trial of the new set. There are plenty of tubes available from a variety of sources: just type "vacuum tubes" into Google. The challenge is finding those that are not only good sounding, but are a deal as well.
I never considered myself expert in the electronic end of audio. My strength was in finding the best combination of equipment to achieve the desired result. That being said, I want to caution against identifying the 2b as "dual mono." It has independent signal paths for each channel, including the volume controls, but it has a single power supply and thus cannot be called "dual mono." The pair of volume controls equate to having a volume and balance control, but require a little more active participation on the user's part. A balance control can be set once and forgotten; dual volume controls require careful setting with each volume adjustment.
Speaking of volume controls, one of the changes between the 2B and 2D was an upgrade in the volume control pots. I don't have specifics, but I have heard complaints that the 2B volume controls tended to be noisy and scratchy. The 2D pots are firm and smooth, even after 20 years of my wife's smoking.
What device is being replaced by the 2B?
Anyway...I hope some of my rambling helps give you insight.
Allen
