Haven't heard the Shindo. I do own the Air Tight, however, and it's my endgame valve amp - to me, the sound of my system is just the sound of music. I no longer feel the urge to find out whether there may be something better out there somewhere. With my DeVore O/93s, it sounds plenty exciting and fiery to me - the exact opposite of an "audiophile" sound, and I even use the very clean sounding LTA microZOTL MZ3 as a preamp!
FWIW, the late Art Dudley was blown away by the ATM-300R, calling it one of the finest amplifiers he had ever heard. He was enthusiastic about the Shindo F2a as well, but he felt that the Shindo worked better with his 16 ohm speakers.
Good luck!