What's the general opinion here of these tubes? They came as stock in my recently purchased Fender '57 Tweed Twin, but the reviews I've read are mixed. I've tried putting a spare pair of JAN Phillips 6L6WGB's in the amp, but cannot get the bias lower than 37Ma before the bias pot reaches the end of its range. I'm looking to get another pair of these tubes which are rated more appropriately for the amp, without changing the bias resistor. I also have a pair of Svetlana 6L6GC's which I intend to try when I have time. However, the Groove Tubes are supposed to be produced on original GE equipment and are the product, I believe, of intensive R&D, so should be very good, right?