Ever since I purchased my Rivera Quiana in 2000, I've been running the same pair of Svetlana (St. Petersburg version) 6L6s and absolutely loved the tone. A couple of months ago, one of them shorted and I decided to replace them with GT6L6GEs. Working with a few different bias settings from cold to hot, I just haven't fallen in love with these tubes like I expected to (based on all the positive reviews). Compared to the Svetlanas, I found the tone significantly muffled and deadened, the kind of tone you would expect from old, dead tubes. Also, I don't achieve the subtle compression that I got, and really enjoyed, from my Svetlanas, even when biased at 70%. Perhaps the GTs are not a good match for my Quiana, but I've decided to put another pair of SEDs (winged C) in my amp in hopes of achieving my old tone. Has anyone else had experience comparing these two brands of 6L6 tubes, not necessarily in a Rivera? What were your opinions?