Jonathan - no not the gradient ones. I don't know what make they are. As far as I know, they are almost turned off - if you know what I mean. Just barely on to reinforce some of the bass. The tweeters each came in a box - and each weighed a ton, OK well actually I'd say at a guess 4kg each. There seemed to be a huge "transformer" type block which was then fronted by the ribbon. There was a switch on the back for attenuation of the tweeter, but no adjustment of crossover freq.
If you had all the gear, and the amps that would stick it (PP valves) then its worthwhile doing it. But if not, then I wouldn't be going out looking for a second pair of quads to do it.
Fran