I used two different tube amps with my Europas/Callistos. The first was Manley Stingray integrated the second were AES (Cary) Sixpacs. Both performed well, with the Sixpacs being my amps of choice for tubes. My Manley ran in ultralinear and the Sixpacs are all triode. Both are backed with great reliability and service combined with respectable performance. It's tough to beat EL84 or EL34 tubes for nice all around sound. Accurate highs, sweet mids and an OK low end. The Manley just left me wanting a bit more at extended volumes, the Sixpacs just excelled in that area. What is the rest of your system comprised of?