As far as tube break in, I believe that you need to have a signal going through the tube in order for all of the tube's circuitry (including pins) to burn in - just turning the amp on without running a signal through it will just burn in the heaters. I went and auditioned a Melos a little while back, and the owner of the amp had turned the Melos on at 8 in the morning, but he didn't have a signal going through it. I arrived at 11 or so that morning and had a listen to the amp, and even though it had been on for 3 or so hours, after listening for a bit there was a marked change in the sound of the amp -it opend up and really began to sing - which reinforces my belief that you need a signal running through the tube(s) for break in/warm up purposes.
The 5998 Tung Sol continues to be my favorite power tube for the Woo, and although I initially liked the 6BZ7's when my amp was newish, I've come to prefer 6922's for now. The stock NOS Philips 6922's are too rolled off and dull for my tastes, and I've got late 60's Sylvania 6DJ8's in my Woo right now and they sound very good - nice and punchy and dynamic and musical, if not a bit soft on the top end. I went ahead and splurged and ordered up a pair of white label PQ USA Amperex tubes, and when they arrive we'll see if they're as good as people say they are.