I own a DSL401 and I love the overdrive sound that an EL84 makes. I wished that Marshall would have gone farther and made a DSL 60 watt model with six, EL84's. (You can actually get 15 watts out of one EL84, but it will only last about 4 or 5 months with moderate playing time.) Yeah, that would be my dream Marshall amp...a 60 watt EL84 amp with at least two channels.
I've known about Yellow Jackets, but never thought about trying them since I have a variety of amps. I would be curious to know what the total power output would be on your 122 with four of them in use? It would probably knock her down more than 20 per cent. 12 watts per EL84 is about the safe max and since you are using the EL34 power transformer, I'm sure you have plate voltage (B+) in the 400 volt range. The YJ's must have a large resistor built in as EL84's do not like B+ over 330 volts. (Do the YJ bases get hot? I mean hotter than what the tubes will make them?)
Enjoy your new equipment. Marty
MUSICIAN'S AUDIO EQUIPMENT-MAKERS OF VERSACAB