Tried a little tube rolling in my Cortese today. I have had the pre since November and have only used the stock tubes up to now. I took out the stock rectifier and put in a Phillips Miniwatt metal base GZ34. To my surprise, I saw no improvement in sound over the stock tube. I then tried a Bendix 6106. after a few minutes of letting the tube warm up, the pre started making noises I did not like (this was with the sound muted). I took the Bendix out and reinstalled the stock rectifier. The pre plays fine. I have not tried tube rolling any other tubes, but it may be that Mick has voiced the pre so well to the stock tubes that NOS is a waste.