The ATH-40X is the cheaper (€ 100.-) brother of the M50(X).
It differs VERY much from the ATH-M40fs in looks, comfort, size and above all sound and is closer to the M50(X) but slightly less bassy.
Above the frequency plot of the ATH-M40X (smoothed 1/6 oct). There seems to be a little quality control / matching issue going on here. I measured it several times and in several positions and even reversed it on the test rig and the (substantial) difference between L and R is really present. 4 dB difference in the 100Hz-400Hz region is a bit strange.
According to some reviews and Audio Technica this headphone is ‘flatter’ than the M50X and indeed it sounds ‘flatter’ than the M50X but still has ‘big’ bass. Personally I like a bit of bass so would not mind it IF there weren’t that much difference between L and R.
Product variance or issue ?
The headphone sounds better than it measures and has a (somewhat fatty and elevated) bass but no ‘warm’ sound. This is caused by the ‘dip’ between 200Hz and 1kHz. This gives voices a ‘clear’ sound and because the FR drops off above 2kHz there is no ‘shrill’ sound. The highs are ‘clear’ and soft but the ‘details’ (the part above 10kHz) is elevated too much but not annoyingly high. The overall sound is ‘good’ albeit a bit on the bassy side and the treble is smooth but a bit ‘artificial’.
Time to investigate and modify.
My efforts are 'logged' here: http://www.mediafire.com/view/y5i5wbnmn43xn15/ATH-M40X_modifications.pdf
With only 2 simple mods the FR has changed to:
The FR flattened out and the HF is much improved. The familiar 'dip' around 3kHz is introduced but as dips aren't as obvious as peaks and dips in that part of the FR aren't that obnoxious the overall sound has improved. Even the L-R difference is much smaller, Highs are very well extended and smooth. No sibilance or harsh sound at all. Bass is good. Bassheads may prefer the stock M50(X) over this,
I never cared much for the M50 and could not get it to sound the way I liked it.
This one does.
For the money (headphone + pads) you get a good sounding closed headphone that can be driven directly from portable gear which doesn't sound 'closed' at all.