I just pulled the trigger on a used 5153 50w 6L6 off of Guitar Center's website. It's the newer one with the concentric knobs in excellent condition. I wasn't expecting to find the new revision used, and at a good price. Now I can directly compare the 50w 6L6 with my 50w EL34. I plan on just keeping the winner, but we all know what will really happen right? Spoiler I'll end up keeping BOTH!