Any amp - it has a 4 position switch to help it adapt to different amp types/PA/Headphone (see manual for details). I've tried the ST with various amps I have at the moment - a Laney VC30-210, Marshall Valvestate MkI 8080 (hybrid - solidstate with 12AX7 in preamp section), Night Train, and AC4TVH - & it sounds great with all of them.
But it will arguably sound best with an amp that colours tone as little as possible to let the modeling really shine through (eg a FRFR keyboard amp such as a Roland JC120) or straight through a PA. (FRFR=Full Range Flat Response)
But I never recommend using a modeling MFX unit with a modeling amp as often you get amp modeling 'conflicts' that can result in mushy tones, & excessive noise (OK if you're switching amp/cabs off & just using it for FX - but that's missing the point of it, of course).


