.... I was thinking that the pancake lens would be the one I would use with the tubes.
I've seen some shots with the 40 and the photog quite liked the match-up. He was using a set of cheapies which I also bought at B&H, non-metal mount, and work just dandy for me but have not used them much - all the better that I did not spend $200 on Kenko or Canon.
B&H recently released a cheapo metal mount option (perhaps Vello) which I would have bought if I had to do it over again as they were only $10 more than mine.
I support the use of cheapo's but be careful of stacking them or at least support your lens.