A.) You're talking about adding a lot more weight to an assembly that, from the way you describe it, is already pretty heavy. I doubt that the wall fasteners used to attach the existing marble shelves to the wall will be able to take it -- Oops!!
B.) And even if additional weight on the lower shelf isn't a problem, even the best, heavy duty industrial slider hardware, has way too much play to provide a solid foundation for a TT (especially when extended.) You would be very disappointed in your lovely turntable's performance afterward.
My suggestion (if that's the best place for your TT) would be to remove a section of the lower shelf, wide enough to accomodate the TT etc., and create an 8" (+/_) deep recess to lower the turntable so it's clear of the shelf above.
B.) And even if additional weight on the lower shelf isn't a problem, even the best, heavy duty industrial slider hardware, has way too much play to provide a solid foundation for a TT (especially when extended.) You would be very disappointed in your lovely turntable's performance afterward.
My suggestion (if that's the best place for your TT) would be to remove a section of the lower shelf, wide enough to accomodate the TT etc., and create an 8" (+/_) deep recess to lower the turntable so it's clear of the shelf above.
