Serge Auckland wrote:

The original 405 didn't provide enough current for anything other than
nominal 8 ohm loads. It struggled a bit even with 4 ohm loads. Given that
a nominal 4 ohm loudspeaker is allowed to go down to 3.2 ohms, and many go
below that, Quad felt they had to provide more current capability, hence the
405/2.

Not to mention a COMPLETE change of the 'technology' of the output devices from
slow to moderately fast.

Graham