Last weekend, a local tube maven brought a small sample of his wares (2 huge tupperware tubs) to a friend's home. The friend also owns an Einstein preamp. Neither of us has had anything but praise for the Einstein in the last year, but wondered if it could be improved even further by changing out the first stage input tubes and the single 12AU7 with something other than the Electro Harmonic tubes provided. The short answer is dramatically so in each case. Almost every NOS tube substitued made an audible improvement. In some cases, a dramatic improvement. Unfortunately the best 6922 variant proved to be a Siemens 7308 from the mid 1960's and the best 12AU7 was a CBS variant from the same period. Each was very expensive, particularly in the case of the input tubes in which 2 are required for each input, so basically over $1000 worth of tubes. The point of this short story is that even the best tubed preamp is probably limping along unless its tube compliment is optimized and a lesser unit with better tubes may sound better. This certainly complicates comparisons, at least, of tubed units.