Here's the site to the amp in question:
What are your speakers, by the way? Do you absolutely need 500 watts? My opinion is based on having and living with VTL 750 and the 250's here for some time. FWIW the inside scoop is that the EL34'd Manley will give more midrange liquidity and inner detailing/beauty than any 6550 design. It may not be completely neutral but is stunningly beautiful and lush without being fat or slow. Piano has never sounded more real than the 250's on WattPup7's. Bass was slightly deficient in triode, but not a big deal as you can switch to linear for more juice. An embedded Manley person once told me that they'd take the 250 over the 500 anyday FWIW.