Kag,
Haven't rolled the input tubes as yet. I have experience with the RCA grey glass VT-231 GT tube from the 50's. I thought it was definately a smoothie, but I actually liked the linearity of the stock EH tubes in my former Cary amp. The Cary definately needed the EH to sound alive. The RCA in the Cayin may be indeed a nice sound as you mentioned.
Ive yet to hear harshness from the Cayin with anything Ive played, except those discs/LP's known to be so.I recently played some trumpet from Rick Braun(Full Stride) and Chris Botti( A Thousand Kiss...) and man they sounded excellent!
Perhaps the RCA/Tung Sol combo would be very good together.
One thing to keep in mind is that the amp/speaker interface is all telling as well as you know.. It's likely the reason why you may have NOT heard the harshness with the Primare.
On my Ushers, if anything, I have a sound presentation that is more relaxed, yet very dynamic with the Cayin. I found that the 4 ohm tap didnt work as well as the 8 ohm tap and by a big margin. The 4 ohm limiting the dynamics and bass. And the Ushers are 4 ohm nominal
maybe try the 8 ohm tap if not being used would also be a suggestion. You shouldn't need the 16 ohm tap unless your speakers are rated as such or very high ohm/hi-sens types.
Just a thought.
Hoping the Tung-Sols arrive here soon!
As a aside note: When I ordered my 88T from AS's..I spoke with Clarke who recommended going with the KT-88 version. He thought it sounded better amp for amp and the 6550 version comes with EH 6550's.FWIW.