I have done a few back to back listening tests with Svetlanas' and TAD's and the TAD's are better sounding to my ears.I am rather fond of the tall bottle TAD's.The short bottles are nice too.
I found the Svetlana's to be too midrangy.JJ's are ok but lack the detail of the TAD's.
Maybe that's why there are so many different types,as many as there are ears to listen.
I would agree with Mike. I like SED 6L6s better than any other current productions I've tried. JJs are good but not as fat sounding or something - they just don't do it for me.
But if your P/T is up to the extra filament draw, try a set of JJ KT66s. I have a pair in my home-brew lo-po tweed Twin right now and love 'em. Fuller lows and smoother highs - a big sounding tube but not too far from a 6L6. In fact sometimes I forget that I have KTs in there.
I have TAD Short Bottle 6L6's in my Top Hat and I liked them more than vintage RCA short bottle 6L6's..... And that's saying something - not that the RCA's sucked - just with this amp - the TAD's sounded better to me. In my '65 Super Reverb - it might be another story. I have RCA's (full size) in it now and I love them....
In my Blackface circuit (1-channel Super Reverb), it went like this: TAD (big bottle)- Sounded like it was aimed at STR Sylvanias. They weren't nearly that "big"-sounding, but their tonality (extended bottom and high end) reminds me of those double getter Sylvanias (STR 387?, I have a pair I like but I can't remember the damn number!).
TAD (small bottle)- Very sweet, somewhere in between my old GE's and RCA's (albeit with less "authority" if you know what I mean).
JJ's- I tried several pairs, and they all sounded good... not great, because they lacked sparkle to me. They didn't do anything wrong, they just didn't have much personality to me. They sounded good, but slightly sterile.
SED- These sounded really good to me (although I stuck with the TAD, big bottles). They were very even from top to bottom but they weren't sterile like the JJ's (to my ears).