Lived with the Westminster and Westminster Royal for almost 10yrs straight ('91-'00). Tried/owned countless tube amps ranging from flea watts Ongaku, Audion, few Carys, few Jadises, many many VTLs (right through their 200w triode double decker Ichibans).
While most mesmerizing tonality was that of the Ongaku's. I too found best served (most musically balanced overall) running large Tannoys with good old push pull amps then - VTL90 Monos (90w triode), 4xWE300B/side. The Cary CAD805 (50w) coming in close second.
True, SETs - Ongaku (especially), as well Cary 300SE, or even Audion, all had in varying doses that seductive 'almost to die for midrange spell' (esp.on vocals, pianos, strings etc.), However, given certain recorded materials/volume level, I often felt that they too (the 8-15watters) are somewhat compromised, loosing control/kinetic feel at times of those two huge 15" drivers.
As such, were I to do it over this time around, a well designed PP triode / OTL / SET (211, 845?) amps *caveat, minimal of 25watts and up will be my preferred choice. *Still fondly remembered of how beautifully 'Ella' sang through with effortless musicality via the 300Bs - 'unforgettable'.. Congrats on those beautiful speakers - post pictures when you get them!