The 6B4G and 300B tubes I could locate from reliable sources were all pretty expensive, so I went with 6AV5 tubes. They sound fabulous, and my only complaint is, my NOS GE 6AV5's are "microphonic". A light tap really makes them squawk, but not a problem in normal use. I drilled my own panel, and added vent holes above the diodes and power resistors. The top and back panels are one piece of metal so I can lift it out of the wood base and set it on the bench as a single unit. Shannon's PCB ensures success. I wired it up, put in a fuse, and threw the switch. After the tubes warmed up, I checked the speaker terminals with an oscilloscope, to verify there was no oscillation, or DC offset voltage. Everything was fine, so I hooked up the speakers and started listening to music. I wasn't certain if there really was a "break-in" period for electronic parts, but the sound improved noticably over the first couple hours of use.