Additional Preamp Tubes

@pindac @thecarpathian @bbroussard @dtorc Thank you for your recommendations.
Early on I read of the favs of King’s for the BHK. Not sure why, but never considered them. 

2 for the Amperex 7316, & strong praise. Added. 
The CV4003 Mullards I am considering are the M8136 version. What does that mean?
BTW, the nice thing about tubes is even expensive ones are cheap compared to components or even cables & can have a profound effect on your sound. Thanks!

Got the answer on the additional designation. 

• CV4003 = British military designation.
• M8136 = a higher-grade military/industrial specification, typically requiring tighter tolerances, lower noise, and longer life.

If they are Blackburn manufactured M8136 (B-code) buying them. If Mitcham Manufactured M8136 (R-code) maybe not  Will find out tomorrow. Thanks 

I went and looked at my tube collection, and forget that I have a pair of RCA JRC 6189/12AU7WA dated 8/62. These are a ruggedized tripple mica black plate tube. I had them in my Rogue RP-1, but went on to the Tekefunken G73-R’s as they were what I was looking for. But in retrospect of this conversion and peoples suggestions, I thought I’d mention them. My pair came in pristine condition from “ The Tube Museum “. Here’s the AI sound description-  Warm , articulate with a punchy midrange and a tightly defined low end compared to standard 12AU7’s. Regards, Mike B. 

The M8137 valves you are interested in are ones I own, where all retained Valves from purchases made, have all been tested and are comfortably within factory specification.

I did not achieve this simply by purchasing valves and relying on the measurements provided by vendors. I only proceeded with purchases when sellers agreed to the testing criteria outlined in my previous post. For clarity, all measurements were taken using a calibrated AVO valve tester.

Over several months, I purchased more than 12 valves sold as matched pairs. The advertisements included attractive measurement data for each valve and half valve section.

However, when I had the valves tested, some of the valves' actual measurements differed significantly from those stated in the ad.

Following a number of returns and further purchases, I eventually assembled a set of six valves that are effectively interchangeable, closely matched, and all meet or exceed factory specifications on both half valve measurements.

Note: A valve specialist who is visited to carry out independent testing, explained that most valves will still function and produce sound even when their performance is well below factory specifications. He also noted that many valve owners do not know their valve's condition regarding factory spec and continue to use low spec valves without concern.

The advice I received was that a valve's published test results shown in an ad are just as important as the photographs used for the ad showing the valve's physical condition. A buyer may decide not to purchase a valve that appears to be in poor condition, and they may also reasonably question the asking price if independent testing shows that the valve's actual performance differs materially from the specifications advertised. In my arrangements, I do not negotiate the asking price when the spec is proven to be concerning; I already reserve the right to return a valve. 

@pindac Great advise & process! 
@buellrider97 Mike! I should have known. This gentleman was kind enough, (a few years ago when I was brand new to this hobby), to loan me several sets of tubes to try. Other than meeting on Audiogon, I was a complete stranger & he shipped me a couple $K worth of tubes (Mullard, Siemens, RCA, Tele’s), not trying to sell them, but to try in my RP1. I ended up buying my Siemens & Tele’s from another source as a result of Mike. What a gentleman! Thank you again Mike!