Not sure about the Bristol lever idea - all those I've seen would be a bit too big & bulky for this job, especially for the clamp under the table, surely?
The Steve Goode and Claymore ideas of making a star nut to increase leverage on the "wing nuts" seems a good idea, BUT you should not really need a lot of force to clamp the blades.
Is the "blind side" nut (opposite to "wing nut" end) of each clamp correctly adjusted?
Is the end of the "blind side" nut (where it meets the blade) flat?
Have your blade ends got a bit oily/greasy by any chance?
Personally I don't really like the idea of changing the "wing nuts" to socket head screws. The whole idea of the Excalibur system is that blade changing is tool-less (from experience of my previous machine, this is MUCH more convenient than having to use a separate tool to un-clamp/re-clamp blades if doing a lot of cut outs). But if you do want a decent Allen key set anyway, personally I'd recommend Bondhuus.
Finally, don't forget we're talking about steel male threads into "only ali" female threads here, so if you apply too much pressure you could strip the thread in the (ali) clamp body.
HTH