Impedance of the ATH-M50 is 38 ohms. If you stay with low-impedance phones, the MOSFET-MAX is absolutely the best bet.
One thing you should try to do - order the Toshiba JFET's from
http://www.bdent.com I used to make up and sell a matched quad, but my supplier ran out of the 2SJ74's. BDent has them at a reasonable price for someone building an amp, but they're way too expensive for me to re-sell that way. You can read up on the AMB-JFET mod to the MOSFET-MAX here (near the bottom of the page):
http://www.diyforums.org/MOSFET-MAX/MOSFET-MAXhistory2.php
Details on how to mount them are here:
http://www.diyforums.org/MOSFET-MAX/MOSFET-MAXmosfets.php
These can make a huge difference in the response and linearity of the MOSFET version at QB2 and QB3.
The MOSFET-MAX really shines with low-impedance phones, but will do a pretty good job with high-impedance, too. The limitation is the tubes, but a a pair of 12AE6's will ensure enough voltage swing for the higher-impedance phones. The key is that the Class A bias on the MOSFET output buffer is at least 125ma and can be pushed to 150ma or more (carefully). If the phones will use it, it has the capability of supplying perhaps 1-1/2 Watts. The BJT MiniMAX version is best all-around, but is really optimized for size, convenience, and the custom-machined case. On the other hand, the full-sized V1.2 MAX PCB offers the flexibility of the onboard DAC and more flexibility with the cap sizes. I usually steer people toward the MOSFET-MAX for low-impedance and toward the MiniMAX if they want to try all phones and are looking for a complete kit.