Thank for the replies all, very helpful. Think I'm going to go with the MB500/800 as I want to keep things light and find a way to implement my pitchblack tuner and B7K into the rack...Ideally in a drawer of some kind, but if they're really heavy I may have to settle for a shelf. Will Velcro alone secure them on the shelf? I'm thinking Bass - Line 6 Relay G55 (racked) - pitchblack tuner - B7K - MB800. Also thinking of somehow fitting a power strip on the inside of the rack so I only have one power cable instead of 3, anyone done that?
Why not just go with a 2U rack and Velcro the pedal to the top of the rack when in use? You'll save yourself a figurative ton of weight without the drawer/shelf and the smaller rack.
Or even consider a small rack bag?