Pues vas a tener razón. En la entrevista de Audioholics a los ingenieros de Anthem les hacen esa pregunta:

Audioholics: ARC comes with a calibrated, serialized, USB microphone. Are there any limitations with the microphone's measurements or does it measure accurately from 20Hz - 20kHz?

Nick Platsis: It is reliable up to 5 kHz. After that it starts becoming too directional with increasing frequency. Maximum correction range for MRX receivers is 5 kHz. This is the normally recommended setting, and the default, in all versions of ARC and is user adjustable down to 200 Hz. In AVM/D it can be set as high as 20 kHz, but effectiveness of correction at these upper frequencies often depends on whether the tweeters (and mic) are at ear level. A listening test is the best way to find out.

Vaya cagada. No les hubiera costado mucho implementarlo en los MRX, vamos... no les hubiera costado mucho "no capar" a los MRX.

Entrevista completa:

http://www.audioholics.com/room-acoustics/anthem-arc-room-eq-interview