Marco,
I too have a pair of 15XR's that I picked up for £30 from a local 'boot sale! I intend to use these in a second system that I'm putting togther which is going to be valve based. Have you any idea what modification you'd like to do to your 15's? Me, personally, I'll probably be replacing the binding posts, internally re-wiring with pure silver cable (cotton dialectic), the foam looks quite good in mine so i'll keep that. Probably re-build the cross-over.

