Rwwear Ill defer to your expertise as youve actually used a tube pre with your 700cx and I decided to look elsewhere. But this was my exact dilemma just last month. I was looking at a tube preamp, either the AR Ref 3 or VTL 6.5/7.5. I contacted the preamp manufacturers as well as Krell and was told by both that in order to properly protect my FBP700CX I should absolutely engage the coupling capacitors when using a tube preamp. Unfortunately placing these caps into the signal path degrades the sound and largely defeats the design purpose of the FPB amps. It wasnt that the preamp was going to leak DC constantly and at a constant rate that would necessarily allow an easy meter read at the outputs. It was that tubes are not the most stable device in terms of consistency; they change constantly in small amounts over their operation period and more radically over longer periods of time. This can cause spikes to occur during lets say turn on until the circuit settles out, this can be enough to fry an amp or a pair of speakers in the right circumstance. I was told that the later designs have much better turn on, biasing, and dc offset circuits then they used too and to that end the risk has certainly been minimized, but not eliminated.
If you do want to engage the coupling caps I have the procedure from Krell for all the FPB series amps. I can send it to you offline or you can contact Krell directly. They were very helpful and had lightning fast responses.
Let me know what you decide and how it works out. I may revisit the tube preamp dilemma in the future but for now I opted for a KCT.
Dave
