The C6b base is for matte printing only.? I buy glossy base (wj824, from STS Inks) for PK dilution.? For matte, just use MK with the PK dilutions.? In fact, you can use the PK as the?deepest "midtone" for matte paper.
I suppose the C6B base would dilute the PK, but it would lose its glossy compatibility, particularly in the higher dilutions.