Just got two pairs of 6FD7s in, one set being American military fat bottles and the other Pro/Comm skinnies. I've tried both sets with the Sovtek and the Sophia Princess. Wow, lots of bass. Bloomy waves of bass. A little more gain than the 6CY7s. Not sure how to describe the differences but I would say very tubey sounding, saturated. I personally like it very much but I could see people saying it is too much, with 6CY7s being more balanced and faster. Even with the gain I don't hear any bit of noise from the WA6 with either the Denon 7000s or the Senn 650s. I'll be sticking with this combo for a while. Still waiting on the Brimar. Volume knob is now staying close to 9 o'clock (or less) when it had been more like 12 o'clock with the 6DE7s.