Quick Report This amp is medium sensitive to a tube swap; maybe not as sensitive as a WA but you can actually here a real audible change unlike my experiences with SWR or a 3Pro. I have lot of older used 12AX7 tubes. During a really long test with a WA awhile back, I found my favorites were: Telefunken, Amperex, Mullard, and National (long plate and does not have nearly the reliability of the other 3). I also checked these against the stock JJ tube. This was the order of preference in the WA and remained the same for the 7-Pro. The Tele just had a bit more detail and air; a clear soundstage and 3-D The Amperex sounded great but slightly different. Maybe not as wide but a slightly tighter bottom (insert joke here). The Mullard seemed to have a more focused sound and not as 3-D; not a bad thing just not my thing. I like the National tubes but I have been told that they are spotty on performance (mine are NOS from the later 70's early 80's). I think these can be found cheap and the others are about $40 each if you buy rebranded tubes like Dynaco, Eico, Baldwin, and Fisher to name a few. The real deals are just a bit more. What's cool with Dynaco (made by Tele) is they ran lower voltages than many applications so one might surmise the will last longer although mine are from the 60's. The J&J was fine but not as refined or detailed. It should be noted that this tube had more gain than all the rest. It also has really short plates. I'd welcome any reports that others have found with a preamp tube swap in a 7-Pro.