Does anyone care to ask an amplifier designer a technical question? My door is open.

Roger,
I recall you saying at Burning Amp a year ago that you did not like 6SN7 tubes in preamps.  I have a preamp that uses these tubes and it is the best I have ever heard in my system.  What is the problem with these tubes from a designer's standpoint?  Thank you. 

Oops, clio09, I meant to say ct0517 above. I keep conflating you two!

@ramtubes. Thank you for your reply to my questions.

You asked: “If you have a good preamp, why change?”. I’ll list a few things that have me looking elsewhere. 

1.) I bought this preamp new, and after about two years it developed an issue. Every few times I switched sources, the left channel would fail to output sound. After cycling the selector the sound would return to normal. I called the manufacturer for advice. They told me to spray the switch with contact cleaner, which I thought shouldn’t be necessary for a two year old unit, but I did as they suggested. While it was open, I noticed that there was a screw missing from the vertical board that the selector switch was attached to. When I rotated the switch, the board moved around. I installed a screw in the corner of the board where it had been missing, and it no longer moved when I rotated the switch knob. That seems to have solved the problem, but made me wonder what else could have been overlooked. 

2.) I recently bought an inexpensive, but well-reviewed phono preamp for another system. I decided to see how this stand alone phono pre compared to the built-in phono stage of my main preamp. The inexpensive phono preamp, through the tube linestage of my main preamp, sounded noticeably better. I asked the manufacturer, as diplomatically as possible, why this might be. They told me that the tube line section of the preamp is much better than the built in ss phono stage. Why include a phono stage that you know is inferior to the rest of the unit?

3.) I’ve read that the 12au7 is not the best tube to use in a preamp circuit. My preamp uses all 12au7 tubes (4 of them), and I’d like to try something that is designed around a different tube type. 

4.) This preamp is made by one of the manufacturers whose amplifiers left you “not impressed”.

I like the way my system sounds, but for the reasons listed above, I’m feeling like replacing this preamp could allow the whole system to sound better... especially after the surprising phono preamp experiment. A system can only be as good as its weakest link, right?

@bdp24, well that makes more sense. We are not crossing over the Quad in the test set up. However, in our respective RM-3s both Roger and I use 4th order Linkwitz/Riley crossovers, 24 dB slope cut at 100 Hz for high and low pass. Roger also adds some EQ to the low pass board that I believe he explained here or perhaps in another thread. In any event I have used the Quads with the RM-3 and my bass array in a biamp set up and like that arrangement, which I also do with my modified Acoustats. I don't run the satellites full range.