I do not run a Feliks but have heard nothing but good things about the amp. A tube I like with my amps is the 1963 Melz 1578 as far as excellent mids, highs, and and accurate bass. There should be a few more folks on here that can talk about this tube so mine is not the only opinion. I just lost a PSvane Mk-2 yesterday when it was knocked to the floor, still in its original box thanks to my cat Finnegan.
I liked it all right but it was not toward the tops as far as overall accuracy. I run the same RCA 6as7g's on occasion but I prefer Tung Sol 5998's myself. Add those to an Incubus amp along with the Melz tube and Focal Utopias and I am a happy man. I could lend you a couple 63 Melz to try but I do not know you as well as others on here so details would have to be worked out. No offense intended of course.