NOS tubes can bring levels of sound out of units that spending 1000.00 sometimes cant.I just put in 2 cv4003 mullards = 12AU7.I replaced the stock Sylvania that came with my Rogue 66 preamp.They where nice quiet tubes,I new i could do better and the NOS mullards have been a God send.Better bass ddead quiet and more clarity and space.If you use a tube preamp.Give some NOS a try you will be amazed.WWW.thepatrsconnection.com is a good source.Kevin at Upscale Audio is also very good at selecting the right tube for your application.
