To say that 7308's is your best bet, as a blanket statement, does you a disservice, Hiendmmoe. There is a variety of tonality and quality among the 6dj8, 6922, 7308 family AND variety within each type. In other words, some 6dj8's will sound better then some 7308's. Some 7308's better than some 6922's. Some 6dj8's better than some Cca's, etc. To say 7308's is your best bet is like saying General Motors is your best bet when the range of choices is Vega to Corvette.
Everyone has their particular favorites and, like a lot of other things, some of these tubes are component dependent. Not trying to sound non-committal, but this is a fact. I have heard, though I don't have experience with the SL2, that good, early vintage Seimens 6922 CCa is a good match with this piece. As a quick and informative primer, check out the AA site, click on FAQ, and read Joe's Tube Lore. I don't agree 100% with Joe's ideas but he gives some good information on the variety among and within types.
