Well, out of curiosity, I tried stacked 1.4 converters with my 400 DO lens. With one converter it is great but with two it was rubbish. Glad it was only an experiment and not shooting anything important!

I use the Tamron SP 1.4x converter....which I believe is essencially the same thing as the Kenko pro series one. The 2x on the other hand produces very mixed results.
Intended primarily for use with my 300mm 2.8, I would be interested to know whether going for another 1.4, 1.5 or 1.6x unit and fastening to the 300mm +1.4x combination is going to be a more successful set up.......otherwise it could be more favourable to go with something like a TC301 2x converter.?
AF is nice, but is rubbishy slow with the tamron 2x (tho still reasonably quick with 1.4x), so MF isn't such a major chore.

any thoughts/findings?

Most things that I have read say that 2X converters don't really do a very good job, at least on Canon. There has been quite a lot to comment on the Canon site at differnt times and the consensus seems to be use no greater than a 1.5X. I'm talking only about single teleconverters not stacked.

Yes, 2x seems poor, but my point is, are two stacked 1.4x converters a better idea than using one 2x converter. Theories of more glass elements seem to contradict themselves.
Is it the lesser of the two evils? assuming that by using one 1.4x converter isn't enough?

As already stated, I have tried stacked 1.4's v 2x a few times and there is nothing in it IMO in fact if anything the stacked tc faired slightly better IQ wise. Not that I am saying that either combo is a good solution in the field.

I did see a thread once that quoted distortion figures for a 1.4 tc and a 2x tc and according to the figures stacked 1.4's should show less distortion.

What I always find with converters on my 400mm f5.6 is that they perform very well if you are close to your subject (even with stacked 1.4 and 2x, see my earlier post) but lack detail for a distance subject which kind of defeats the object for me.