Help Tannoy XT6F with Primaluna

Hello all.
Just got a pair of Tannoy Revolution XT6F speakers. They are supposed to replace KEF LS50s. I'm driving them with a Primaluna Prologue Premium power amp. The new Tannoys sound awful with the Primaluna. Very peaky upper midrange and not much bass. Pretty unlistenable. Tried them with my old Rotel RB990 solid state power amp and they sound much much better. Tried the 4 and 8 ohm taps. Tried the stock Primaluna tubes and some KT 150s. No good. The Primaluna sounds great with the KEFs. Any ideas? Thanks in advance.
Ben