A few tube enthusiasts have opined to me that they found the Ken Rad 6C8G (RPs) to sound fairly similar to the (famed) Ken Rad 6SN7 (RPs). Now, I've haven't had a chance to actually hear the highly regarded 6SN7 version.. but if the sound of the 6F8G variant is any indication, the 6SN7 version must certainly be something special. The 6F8G's bass is definitely punchy and impactful.. w/ the focus being on the sub bass rather than the midbass. The treble has a smooth, balanced (in reference to the midrange) presence and good depth Treble is crisp & complimentary to the addictive bass and deep, engaging midrange. It's certainly in the top two or three favorite tubes I've rolled into the Crack.
I agree 6F8G are terrific tubes, electrically identical to 6SN7, so there shouldn't be any reason for them to sound too different if at all. There are a lot of different ones out there, I have tried around 5-7 different brands. In my experience VT-99 military labeled brands being my favorite. Will need to check my listening notes for more detail.