EAR 864 V1 V3 Tube Type

I forgot to update my info.  I changed from the Sylvania (two went dead one year apart) to a Mullard CV4004.  It is a military tube and has no relation sonically to a typical warm Mullard.  It has an extended frequency response at both ends, super dynamic, clean, clear and neutral but full bodied mids.  I love it!  It is also very expensive as an NOS tube (bought at $125 back in 2010).  However, there is an apparent new version that is cheap from Upscale Audio.  Have not tested that one.  

To build on what Fleschler stated, the ECC83/12AX7 has a gain of 100 while the 5751 tube has a gain of 70 which is a decrease of 30%. This means that while the 5751 will probably be less strident and possibly more transparent, your power amp will have to work a bit harder to get the same loudness level as the ECC83/12AX7. Many guitarists substitute a 5751 for a 12AX7 in their amps because they like the mellower sound it produces. Moving along, the ECC82/12AU7 has a gain of about 19 or 20, which means that if you placed the ECC82/12AU7 in the socket that was meant for the ECC83/12AX7, you certainly would hear the difference. By the way, you might want to try Telefunken ECC83/12AX7's, smooth as silk. They are marginally better than the Mullards. Hope this helps.

In my system, I use a clear-top RCA 12AU7 and a 1950s D getter, ridged black plate Sylvania 12AX7. Other systems could use other combos but the RCA is easy to find and relatively inexpensive. It has great bass and dynamics. I found the Mullard 12AX7 very good but less balanced for my system. Some use GE triple mica lower voltage 5751 tube type which is extremely detailed sounding (I prefer less resolution and smoother/warmer sound). Enjoy.

P.S. I don't like the stock tubes at all. Brittle and bright sounding, yuk.

From my experience, the ECC82 has a greater influence, but YMMV

Thanks guys! By the way, I wrote EAR and replied me that V1=ECC83 and V2=ECC82. May I know which affect the sound more? V1(ECC83)? or V2(ECC82)? Thanks again!

The tube closest to the front is the 12AU7, when I had my 864, after much rolling of NOS tubes, I ended up with an awesome combination: Tesla (Not JJ) ECC802S in the 12AU7 position and a 60's Amperex 12AX7A...have fun rolling !

Anyone have contact information for Mitch Singerman?

I have 834 Int so can't help.But I think they might have it shown in manual you can check online.Either that oir I am confusing it with CJ or another tube piece I have had.It would be easy to email there tech google Mitch Singerman who does all the warranty work for EAR in USA and ask him about tubes your using,if he'd try others etc.
Cheers
Chazz