I've been using a pair of the larger BE 15's for a couple of years and have worked with their 212 before that. Bag Ends are very flexible as to the power it takes them to perform - I've used them with low powered heads and thermonuclear rack rigs. Always got good results.
Never had one of the 18s, but a friend had one for quite a while. Sounded good - he found it a little boomy with the front port open, so he tended to keep it covered, operating the cab sealed. To me it sounded fine both ways. My only recommendation would be to put some casters on the 18. BE cabs are very heavy. You could use the 18 as a trolly to carry other cabs (or just tow it behind your car like a boat trailer
). Good luck