I have also install screen stoppers for the output tubes on all the amps and also installed diodes on the rectifier tube on all the MKIII's.
The two MKIII's , drive the Altec 802-8D drivers mounted to the 511B Altec horns and the only way that the Altec Horns, quality of sound was vastly improved, I had to take exchange the JJ output tubes with the Golden Lion Series tubes. When I did that, the best way I can say it was that the bandwidth also improved, the Altecs are a pleasure to listen to, and can really add to a large system . The other two MKIII's using the Golden Lion Tubes are bi-wire and drive the JBL,L7's high frequency part of the speaker that includes the 8in woofer and I noticed a considerable improvement in the upper bass and midrange.The 12 in woofer is driven by a SS amp.
The air filtered box is in the basement and all four Dynaco MKIII's are in the air tight box with furnace filters on one end and a fan on the other side, this forces clean air over all four amps, that run much cooler, then what they normally would.