I took have the Fostex T50RP. When the Alpha Pads came out, I bought a pair of the pads from Mr. Speakers and put them on my otherwise unmodded T50RP. I too have heard the Alpha Dog, at the Dallas meet last month, and I too have compared the three -- Alpha Dog, Mad Dog (both latest versions), and the T50RP with Alpha Pads. And my conclusion is exactly the same as yours. The Mad Dog is very similar to my T50RP in sound presentation, particularly imaging, with a better frequency response. The Alpha Dog to me sounded very different, and much more coherent, especially at the low end. I spoke to Dan while at the meet, and he is very good at what he does -- he explained to me several things that are wrong with the T50RP design and what drove him to the "transplant" solution that is the Alpha Dog. When I put on the Mad Dog after listening to the Alpha Dog, I said to myself, "This is like listening to the T50RP" -- very familiar to me.
Right now, I feel that I can get my T50RP with Alpha Pads very close to the Mad Dog through some EQ. I can't change rates of decay and resonances, of course, but the EQed sound is very very good. But I still am looking for that coherent low end. I feel either the Alpha Dog or the new Audeze LCD-XC might be in my future.
I did try the new Audeze LCD-XC and it was heavier than the AD and at three times the price of the AD I was not too impressed by it. If I was able to borrow the Audeze LCD-XC over to the table that I was listening to the AD with the same amp and source I would have a better indication of the sound quality. I like the HE-6 much better than the LCD-3.I would be happy and content to stay with the Alpha Dog even over the HE-6.