Y cables or real splitters?
Y cables.
I think the main thing to come away with here is there are variables like what preamps and what mics you will be using.. Sometimes you can get away with it sometimes not.. In a case like this I say its better to spend the extra cash and get something that will do the job. If you have the ability to really evaluate the changes going on in an objective manor then you can always try a Y cable..
Yeah, this has me curious to learn more from the people who have noticed a difference to see if there are any commonalities -- eg, transformer mics or not, perhaps electret or not, high or medium impedence recorders, split mics into preamps or recorders, did the split mics go into two different preamps and/or recorders that might have different input impedences or did they go into a single 4ch or multichannel recorder, etc.
I think I have only split a mic signal once or twice (more often have split a soundboard signal into multiple recorders, or split a line-level preamp signal into multiple recorders). I don't remember hearing a difference or noticing something off, but the effect may have been there and I hadn't noticed it.