I found the JJECC83S valves to be more dynamic in the phono stage of my 25R, than the NOS Mullards, TJ Full Music, and Genelex Gold Lions I tried at the time, and recall reading somewhere that the JJ's have higher gain than the standard 100, so thought at the time that was the reason for what I heard.

I must confess to swapping out the supplied regulation JJ's, for the NOS Mullards i already owned, before listening to them, so will give them a try this week.

I recently replaced the supplied JJ ECC83S with a NOS Mullard M8137 (CV4004) in my Series 7, & that increased the dynamics & punch a notch or 2, or maybe what I heard was reduction of the noise floor