For what it's worth, i have been using the Sennheiser PXC360BT (which i understand to be somewhat similar to the MM550, just without the mic) for a bit now and am loving it.
Havent heard the K400s personally, so maybe you could describe it's sound signature in greater detail for the uninitiated. And the genre of music you primarily listen too would help a lot as well.
I had a channel imbalance on my SR-507's after just a couple of days of use. Did you ever find a solution to the imbalance or your 507's, or did you have to send them back? I'm a little worried about the RMA process as I purchased mine through a Japan source.
I'm getting started with an old STAX classic, the SRM-1/Mk2 PP that Spritzer sold from his collection just a while back. And having compared the SRM-1 with the SRM-007tii (stock) at an audio shop, i didn't feel i was missing too much but adopting the older STAX amp.
Sorry to say i've no other experience with those amps you listed (SRM-323, SRM-006t or the works).
I had a go with the SR-507s and the O2Mk2 recently and i primarily went with the SR-507s because i felt getting the SR-507s weren't gonna be such a huge compromise as with the earlier lambdas and didn't wanna totally commit into the O2s with the C32 looming around the corner.
I had a brief comparison with the SR-207s as well and the 507s were definitely fuller and more mature sounding than the 207s but haven't had a go with the 407s yet which i read to be really close to the sound of the 507s.
Anyone had a go with the 507s and 407s or even 307s for that matter?