+1 email VTL to confirm but 6550 and KT-88 have “similar” bias requirements. The Series III can run either per website. Beast of an amp, btw. If KT-88 equivalent, consider looking at Shuguang Nature KT-88-T (not -Z).
I have a Dennis Had Inspire KT-88 that is designed to run 6L6, 6550, KT-88, etc. Stock tube was GL KT-88, good tube, clean, transparent, linear, up and down frequencies. The Shuguang KT88-T has a bit more heft in bass and depth in mid-range. Treble extension is equivalent. Kind of like the difference between a Porsche Carrera and Carrera S. The S has a little more grrr at every turn.