Hi there, I hear that Beolab 18 has been made in several versions: Mk1, Mk2 and Mk3. What is the difference between these versions in terms of sound quality, connectivity, and future maintenance (spare parts)? In particular I wonder what version the type numbers 6807 and 6676 belong to? Is it advised that, in a surround setup, only certain versions are used? Glad to hear your view. Greetings! Frank