Other Stax amps aren't real upgrades over a modified T1.
If you want to move upwards, you are looking at aftermarket amps. A KGST is something that I would most like to compare.
Seems like a nice step forwards according to many while keeping some of the warmth in the sound that Stax hybrid amps are known for.
Recently acquired a KGSSHV Carbon, which is a whole different league with regards to power, but it's also a different sound.