I have a friend who lent me a DB680 to try out before buying. He said he hated the sound... too muddy and the Power bulb kept burning out. I got it and opened it up to try some fresh tubes and to clean it out and to my surprise found only 4 tube slots instead of five.. only 2 12AU7s instead of 3. I swapped out the ancient looking AX7s and I immediately heard it to run quieter and it didn't sound as muddy any more. I've got a pair of 12AU7s on their way, but all the wiring diagrams and pictures I see show 5 total tubes. Anyone ever heard of a different layout? It has a really low serial number and I'm wondering if it use to be built differently or or if this maybe have been modified by a past owner. As a side question... does the 12AU7 Tubes really effect the sound that much?