i have not heard the bakoon amps. but as for the Zana and GSX, its about as good as it gets.
the Zana was bright but also very transparent and had a huge soundstage that really got rid of any closed in-ness the TH-900 might have. it made it really neutral sounding and the bass was more neutral, if you like the TH-900 as a bassy headphone, this might be a bad thing, but overall i think most people who listen to well recorded stuff and need a closed headphone would really enjoy the zana.
the GS-X sounds amazing too. warm, still having good bass and overall sound, just alot cleaner, and more neutral than th-900 off regular amp. well rounded and overall fantastic. about as good as it gets as well.
my woo amps WA6-SE and WA2 i used to use TH-900 on didn't sound as good. but the WA6-SE was really close to gsx. just one thing i've found. unlike the older denon series, the TH-900 is more sensitive, and extremely sensitive to an amps output impedance. i noticed the higher output impedance of the tube amps makes the headphone really bassy (especially the high imp output of the woo amps) . like it was a bass monster, like the bass even fatigued me, and im a bass head. but it did sound good on some recordings, and the bass was still tight and strong just way too powerful. it just filled your head with sub-bass. it had recessed mids though.