Tube Rolling for Convergent Audio SL1 Preamps

just wondering if anyone has experience in experimenting with different tubes in a convergent audio sl1 preamp....ken stevens, the preamps designer, will tell you that premium nos tubes like old telefunkens, mullards, amperex, etc will make NO DIFFERENCE in the sound quality....that his design works with todays current tubes just fine.....i find that hard to believe since they can make a HUGE difference in other pieces of gear.....does anyone have an opinion that they can offer?.....many thanx!
I have a Cat SL1 mkIII and did a little bit of NOS (telefunkens) rolling and would agree with Ken. I had much more noticeable results tube rolling on my power amp input (and to a slightly lesser degree, power) tubes.
why do you suppose that is? what does ken do that other designers don't? is there something fundamentally flawed about a design that is affected by the quality of the tubes?
I guess on different days, Ken has different opinions.
He has told me that Telefunken smooth plates are the best he has heard in his preamps. I think he hasn't done much experimenting. There are lots of tubes that will vastly outperform Telefunkens in my CAT Sig MK2. I am currently using Amperex 6922 pinched waist and Sylvania 5751 blackplate 3 mica square getter tubes in the pre. This combination is the best I have found (lots of rolling) and very expensive. Almost anyhting will outperform the Sovtek 6922 that is the stock tube in the pre. There are lots of tubes that will outperform the EI 12AX7's. If you try the inexpensive American tube types you will find better tubes than the stock tubes.

Just a quick correction on my previous post. I have the CAT Ultimate MK2 not the CAT Sig MK2.

I have a SL-1 Ultimate Mark II but have previously owned a Reference Mark II and a Signature Mark III. And tube rolling definitely makes a difference. I have spoken to Ken in the past and his response was that the stock tubes made the preamp sound the most "neutral", but that the sound can be tailored by using NOS tubes. I also think Ken is insuring that the current production tubes will always be available, whereas NOS tubes will not be. I found the best tubes, to my ears at least, to be Siemens CCa's for the 6922 positions and Raytheon D-getter 7729's (just try to find these!)for the 12AX7's. I tried some 5751's and didn't like the lower gain and reduced dynamics. Ken doesn't recommend these and I can see why. Others seem to like these but I don't know why.
i appreciate everyones input....i had a hard time believing that different tubes wouldn't produce different results....your experimenting will point me in the right direction.....many thanx!