I had read many positive reviews of AT's M50 and M30 series. I won't go into details of describing the M30:
Sound quality, design and comfort are all top notch for the price (which at the time I bought these was 55£) Especially the design is a delightful surprise. the cord is thicker than usual, almost rope like in comparison to usual headphone cords. The head band is soft and there is good amount of room for adjusting The Ear muffs are mounted on cylinder-like part which allows the muffs to move around 10-15 degrees around their own Y-axis allowing for nice adjusting to head shape.
The sound quality is, despite all the other points, strongest part of the ATH-M series. There is no headphone in the 40mm size that would leave M30 outside the spotlight. The clarity is best I've ever heard with headphones of this price and the bass is there. I'm not joking you can literally feel the vibrations with less than half the maximum volume.
Technically M30 comes quite close to M50, the design is slightly different but the numbers are quite close. M30 rocks an 40mm element while M50 is driven with 45mm one this translates to around 6k kHz and better Decibels to watts ratio to M50 favor while M30 has slightly higher decibels but requires more energy to reach its best performance.
However, considering the price of M50 is almost double of M30 at best M30 is definitely choice for home user who is looking for the studio quality.