Signal tube versus power tube, sound impact?

@djohn - I have a Herron as well. Much depends on the system synergy. Picking good quality NOS tubes from knowledgable folks like Andy’s Vintage tube service, Brent Jesse and the like - who can match really quiet samples will pay dividends. Myself I’ve got a quad of 60’s Telefunken 12AX7 because of the low output MC cartridges I use. The 5th tube is always a 12AT7 type, and using a premium E81cc, or a ECC801S also produces sonic dividends. (I have a 801) Brent Jesse’s web site has a good generic description of the sonic signatures between the vintage tube makers. (Mullard, Amerex, RCA, Telefunken ect.) For me, if I was to change anything, I’d swap Telefunken 12AX7 with Amperex for a little more mid range warmth. Again it’s about system synergy and your budget. All the best on your quest!

Thanks to the OP for getting this started. I know it’s been argued about over and over and round and round. But I’ve enjoyed reading the posts, including the links.  This all will be settled by approximately never. Yet it remains interesting to read about. To me. 

I would replace to signal tubes first, research and get the best you can afford.  Why they will last.  You will have to replace the power tubes because they burn out much faster.  Just my experience.  

I use JJ KT77s. They sound fantastic on my Primaluna. I have not tried their preamp tubes.