I don't get this. I've got this Stingray fretless, it doesn't do the 'mwah' thing at all. Every other fretless I've had (including the US Jazz I de-fretted myself) has done it, but this one doesn't, despite being more expensive than all the others!
I've had Rotos '66 rounds on it (as I have with all the other fretlesses) and recently strung it with Ernie Balls, still nothing. The action is pretty low and the neck pretty straight. The nut is cut so low the strings are almost on the fingerboard at that end.
Is this a setup issue? I've Googled and Googled, but there seems to be no consensus on what creates that bloom effect on fretless. Hopefully some of you will be able to agree and give me some advice.