I believe it is much more likely that the acetone washes away whatever hygroscopic contaminants you may have present in the crude product, while the ether and methanol does not (this assuming that you are perfectly sure that all your solvents are dry).Good point, this could be a good explanation!
About air humidity and winter: my "empirical data" is indeed from the hottest months of the year (ether seemed the most volatile solvent to me back then). We'll see - just the right season to experiment on that...
Grignard: well acetone is slightly polar, too - or not? But I too think acetone allows for more water being present without getting muddy crystals. So it could well be that it binds more H2O molecules (or does so with more ease) than ether does.
Besides, acetone is way cheaper, easier to handle (not nearly as dangerous) and you don't get that nasty buzz from it...
(think I will try wetting some crystals with ether and some with acetone (both completely anhydrous of course), and see what will be left after evaporating the solvents...)
While we're at it: has anyone experience with using chloroform for gassing? Some amine hydrochlorides are soluble in it, but it could be easily stripped with vacuum, and then, flooding with dry acetone should result in a nice in-situ recrystallization...
indole_amine