6sn7 or 6sl7

There are so many varieties of 6SL7 and 6SN7 tubes available that it is hard to generalize.  But one thing I would say is certain: for a hobbyist, as opposed to a manufacturer, use only vintage Old Stock.  Why bother with modern reissues when superior sounding vintage tubes are available at such reasonable prices?  A mfr needs a large supply of identical tubes.  A hobbyist doesn't need hundreds of the same tube; he just needs a couple of whatever tube sounds best.

For the 6SL7 slot my very favorite is a Sylvania VT-229 (military designation for a 6SL7) black plate from the 1950s or 1960s.  I have several purchased on EBay which sound superb.  My suggestion is to try several varieties of vintage tubes by buying inexpensive "used but still test good" on EBay.  You will quickly form your own opinions about what you like and what you don't.

And I should also mention another favorite 6SL7 variant---Brimsr CV1985.  In the right application it too is superb: fast, airy and detailed and very deep tight bass.  A little hard to find but worth grabbing when you see one.  A real workhorse too.

Make that Brimar, not Brimsr.

For those 6sl7/6sn7 users,don't be afraid to swap 6sl7/ 6sn7 or vise versa,or even 6sl7 on one channel  and 12ax7/ 6dj8 12au7 on the other channel.it won't hurt your preamp.I've done this a hundred times ,I even use 12ax7,12au7.12at7,12bz7 & 6dj8/ecc88 etc. with an adapter with no problem in my socket modified Dyna Pass II.

The absolute best 6SL7 is the Mullard ECC35 CV569 brown base which are rare as hen's teeth. Fortunately, there are the Tung Sol black glass 6SU7 GTYs which sound just as good and can be found if you look. Both are NOS. Both can last 10,000 hours. They sound terrific - very natural.