I have limited experience of MDMA (a few 120-150 mg ish doses a decade ago, and a 80 mg dose recently) and more extensive experience of Methylone (not sure how many doses, but about 7 grams or so in 180 to 250 mg doses, so at least a couple of dozen, I guess) and none of MDA, so I'm sure you'll get more informative answers than this, but I'll have a go at answering, based on what experience I do have...
I'm not sure what you mean by mellow... if you mean non-stimulating, such that sleep is possible, then I'm not sure Methylone is particularly mellow. Certainly, I've never been able to sleep on a full dose of Methylone (180+ mg), however much I want to, for at least 6 hours after dosing (longer, with higher doses). And most of those 6 hours are in the post-peak period, where the characteristic MDMA-typical empathogenic effects (quite recognizably from the same loved-up family of effects, even though undoubtedly there are differences that a more experienced MDMA user would note) have had their half hour or so in the sun, and left behind them the not unpleasant residual stimulation.
I guess a low enough dose of Methylone (like a low enough dose of MDMA) could avoid the stimulation while giving some pleasant effects. I seem to recall from my second titrating dose of Methylone (120 mg) that there was no notable stimulation, just a very gentle and mild but lovely peak.
A friend with extensive experience of MDMA and cocaine said, on snorting a line of Methylone (I'm guessing it might have been about 80 mg, but it wasn't weighed), that it was 'like pilly coke' (i.e. it was like cocaine, but with an MDMA-like euphoric rush combined with it). Not something I'd noticed, myself (not having tried coke more than couple of unremarkable times or so; and also only having taken Methylone by the oral route), but that fits with the known profile of receptor activity of Methylone, which is more dopaminergic and less serotonergic than MDMA.