well the very early Floorboards (RB series) don't send the same communications to L6 devices as the newer FBV devices, so if by FBX you mean the old floorboards, they've not been compatible with new L6 products for many years.

If you're talking about the FBV or sometimes called FBV longboard, then that will work with the Spider Valve MKII.  it's not specifically made for it, but it will work.  The compatibility page for foot pedal controllers is a little confusing, as I think it's more geared to 'if you are going to buy a new pedal.... which one should you buy'

If you look here at the Spider Valve MKII HD100 advanved manual, page 6.1: http://line6.com/data/6/0a060b316ac34f05933ca8847/application/pdf/Spider%20Valve%20MkII%20Advanced%20Guide%20-%20English%20(%20Rev%20A%20).pdf

you will see the FBV Shortboard MKI or MKII or Longboard (aka FBV)  pedals will both work.  as will the FBV express pedals, MKI or MKII.

The same applies to the spider valve MKII 1x12 and 2x12 combos.

I hope this helps

Cheers

Rowbi