Hi France, from Belgium :bigsmile:
Did you buy the two drives at the same time, at the same shop? From your post, it seems both are actually, as you put it, "sick"... you seem to already know your stuff, I don't see what could be tried that you don't already have... well, maybe someone else will have an idea.
Like many other users here, I have no problem to burn @16X with my own 3540A, even though it's now connected as slave. It's the most trouble-free DVD drive I've installed and used as yet (had a Benq 1620, now a Pioneer 109 and this 3540A).
My opinion: take them back to the shop and complaint. Your drives should write @16X, period.