I concur about the API test kit. While it's notorious for giving false positives at 0.25, there's no denying that something is going on to have ammonia in 2 vessels and none in another.
You said you have 6 cats? Respectfully, there's no way you can *not* smell cats when there are that many of them. Do any of them spray? That behaviour would be problematic, no matter how meticulously clean the litter boxes are, because that activity doesn't hit the litter box.
Trying to remember how many cats I had back in the day when I bought a house where the prior homeowner left me 2 pregnant cats (!) ... and I already had a (neutered, male) cat... 1+2+4 kittens and 5 kittens... I got you beat - 10 cats. (Although we rehomed the kittens pretty quickly, and then another litter came along and then we took care of fertility business...)
I digress - the presence of cats in and of itself, and their associated litter boxes shouldn't pose a problem or I'd have seen this sort of thing happen a lot more often. In fact, I had a client who had umpteen cats, whose house would make your eyes water, and she had, surprisingly, no water quality issues (but the air quality in the house was likely a health hazard.)
We may be meowing up the wrong tree, as it were.
Where are (were) the scented oil dispensers, in relation to the locations where the water samples were collected?
Also, do you have a phosphate test? If so, what are the results of phosphate tests on the same specimens?
Jenn