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.