The comparison of these two tubes came up on another thread where I mentioned that my system was sounding harsh in the high end. It turns out that this is largely due to the output tubes being worn out in my BAT-VK5i pre-amp; although some harshness remains. While I was rolling the eight 6DJ8's in the output stage, I compared the RCA NOS 6L6GC's Blackplates with the new JJ's for the power stage. When I initially put in the JJ's I thought, "Wow these don't sound so bad, I wonder what it would be like to have sex with Angeline Jolie..." (The latter type of thought just goes through my head now and then.) There seemed to be a definition in the base that was not there before. However, I noticed what I thought was a slight loss of detail and some los of forwardness in the presentation. Then, when I put the RCA's back in, it was extremely clear that they are vastly superior in terms of detail and definition and the previous loss with the JJ's was much more than slight; although the RCA's are also responsible for some of the harshness, or at least increasing it with the increase in detail. I guess this is often the case, increased detail sometimes includes some hash in the high end. In any case, the RCA's were like lifting several veils and will remain in the preamp. I will be pursuing getting rid if the remaining harshness and any suggestions are welcome. I may try the NOS Tung Sol 5881 in the power stage, as these are commonly used and coveted for this pre-amp.
By the way, this is a pre-amp and is not Push Pull (PP) and does not have bias adjustments.