I am currently using the stock Sovtek 6922 tubes (6 in total) in my SF Line 1 preamp. I have heard that changing the tubes can have a significant impact on sound quality and characteristics. Tom has mentioned that he has a lot of tubes in stock, but I was curious as to whether tube brands have specific sonic characteristics, e.g. Sovtek vs Siemens vs Amperex. Any suggestions, experiences, or comments are welcomed.