Re: Tubes: EL34's vs 6L6's
In general, EL34s have more of an upper mid peak than 6L6s (think Plexi vs Bassman). IMHO EL34s yield more "crunch" while 6L6s have more "chug". Personally, I prefer 6L6s for clean and EL34 for dirty.
The earliest Marshalls came with 6L6s (as they were more or less Fender clones) but Marshall didn't take long to switch to EL34s because of difficulties sourcing 6L6s in the UK.
FYI, though some amps (e.g., some Mesas) can run either 6L6s or EL34s, the two tubes are not necessarily interchangeable in an amp. They require different bias voltages and EL34s draw a lot more heater current than 6L6s.