My 2 speculative guesses:
1. Serotonin regulates (amongst other things) our sleep patterns and therefore it could make you feel tired, especially a large dose releasing all the serotonin.
2. Serotonin is linked with the neurochemical melatonin. Melatonin is responsible for making you feel tired and drowsey in order to sleep. While my knowledge is limited, the increased serotonin could raise the melatonin crossing the blood-brain barrier and make you feel tired.
Just some thoughts as to why we feel floored, stony and tired when MDMA is a stimulant. Its my personal opinion that large doses cause such a large influx of serotonin crossing the blood-brain barrier that melatonin is significantly raised making us feel euphoric, but chilled out and tired. The bit I don't get is that your s/o only took 75mg which is not a large dose in theory. I have a low tolerance to MDMA and have been slightly floored on 100MG. It's known that the dose-response ratio for MDMA is quite eratic. It doesn't simply show an even positive correlation between amount taken and intensity of experience. Erowid seems to suggest that the dose-response curve gets far steeper at 100MG+. Personally I think the threshold is more independent, in my case I would guess that the low-intensity threshold is breached at 90MG and after that the MDMA taken feels exponentially stronger. For your wife, the intensity threshold may be breached at an amount of MDMA recognised to be very low (60MG for example). Hypothetically speaking, the 15mg's of surplus MDMA could cause the over-stimulation of serotonin, increased melatonin and thus the feelings of tiredness.
On the other hand, the low dose taken might be causing an under-stimulation. Since it appears your setting is chilling at home on the sofa listening to music? A non-stimulating, relaxing setting where you would feel comfortable sober would impact the MDMA high and make it more chilled. Might be a dumb suggestion, but just try screaming as loud as you can. It worked for me once when I was floored and brought me back from the dreamy state I was previously in and then I felt far more energetic and euphoric.
The dose-response relationship really interests me and its inevitable that it's far more complex than any of the literature has covered. If only it wasn't illegal and researchers could actually research it more conclusively we could find out more about how it varies. There's no doubt in my mind that the dose can completely vary the neurochemicular balance in the brain and therefore completely change the experience.
I wouldn't question your product, i'd say it is infact more of an indicator of pure MDMA. Adulterants are typically reported to make it feel more speedy, trippy or weak for example. Rarely are adulterants reported of making MDMA feel more relaxing and induce tiredness, but it's not impossible I guess.
My recommendations would be to try experimenting with dose and set/setting. Go to a music event (or something more stimulating of your choice) you would enjoy sober and try giving her 75mg's and see it's still a lethargic experience. If it is, you could conclude that it's not the setting and it's the dose. Try more or less in a similar setting at a later date and just see how she reacts. Remember mindset can play a part in the experience, if she is expecting to be tired from the MDMA it could influence the tired feelings in a sort of placebo effect, try the screaming technique to see if it changes anything.
Hope some of this helped.