That award-winning formula is at the heart of every one of our professional headphone designs: M-Series for closed-back monitor headphones, R-Series for open-back reference headphones and E-Series for in-ear headphones. Professional Headphones Online reviewers, top audio professionals and cult followers have long agreed that the ATH-M50x is an unmatched combination of audio and build quality, whether in the studio or on the go. During product registration, enter only the last 7 digits of the serial number, excluding any prefixes. How can the answer be improved? The serial number can be found beneath the right ear pad, exposed by separating the magnetically attached ear pad. Or, contact the owner AUDIO-TECHNICA CORPORATION of the AUDIO-TECHNICA trademark by filing a request to communicate with the Legal Correspondent for licensing, use, and/or questions related to the AUDIO-TECHNICA trademark. Write a review about a product or service associated with this AUDIO-TECHNICA trademark. Buy with confidence as the condition of this item and its timely delivery are guaranteed under the 'Amazon A-to-z Guarantee'. Buy Used and Save: Buy a Used 'Audio-Technica AT2020USB+ Cardioid Condenser USB M.' And save 61% off the $269.00 list price.