I have used mj15024 transistors for the top three transistors (those are the outputs, the bottom one is the driver) and had very, very good luck with them, and they are quite available, and will not break the bank. make sure you have the mica insulator under each one, and use thermal compound.
the 15024 is a much more robust part, in mine I removed the 2n1304 and 2n1305 I believe they are, from the driver board, removing the fold back current limiting. That circuit in its own is known to blow up 400's when driven hard, becasue of its loop response time, it may turn on both sides at once.
with those transistors, and no current limit, my amp has been driving a 2 ohm load for over 5 years without a single problem. with a huge amount of power.
downside of no current limit however, you short the speakers leads, and nice things are not going to happen, so do this at your own risk. ymmv....
there is a newer series of mj transistors that are arguably even better than the 15024's, they are mj211xx, forgot what the two last digits are, but for a 400, you want the npn varity.
I have been very happy with all my phase linear amps over the years, don't let anyone convince you otherwise... the newer output transistors really wake them up, and make them much more bullet proof. on ebay,you can also find matched output transistor kits, cap kits, driver board transistor kits, led meter light kits, about anything you might need.
Good Luck!
Brian