Quad did cover the alternative use of the 5883 and 6L6G in its Quad II instructions, as shown here:
The 6L6G was specified to be used in matched pairs. I imagine that this may have been because of its lower maximum anode dissipation as compared with the KT66.
I recall that back in the early 1980s, the then NZ agents for Quad put a 6L6G pair in one of my Quad II amplifiers when they were serviced. As I recall – and as expected – in normal use I couldn’t tell the difference between that and the other one which retained its KT66s. Perhaps there would have been a difference when driven to clipping, but I didn’t go there.
Cheers,