While stereo Bluetooth headphones can be convenient they really have horrible sound quality. The biggest problem I've had with my two Motorola sets is the pitch modulation that occurs. It's especially noticeable in accoustic music or music that doesn't use a lot of distortion. It's really horrible to me, maybe because I'm a musician, but anyone should e able to hear that there is a problem.
Anyone else notice Bluetooth pitch modulation?
I consider myself an audiophile, and I've yet to hear pitch modulation from either of my S series headsets.
Then again, these are not just for listening, they are also for communicating - both include a mic and controls for answering, making and ending calls as well as basic music controls (next / prev track, start / stop / pause). That may be the deciding factor, though I doubt it.
Since you are a musician, would you mind referring me to a track that I can compare with? Also, what device are you listening to the track on, and have you used the same headset with your computer (meaning that the modulation may not be because of the headset itself, but because the device you're using to listen to the music may, in fact, be having problems keeping up with streaming the music via BT to your headset, and thus the modulation...)
The reason I say that is that with my old Motorola DROID, running CM7, I had very little system memory left - and I noticed that if I ran Pandora, especially on high quality, it did almost exactly what you are describing - songs I know by heart chord for chord would be out of tune, at times as much as a full step, and I narrowed it down to Pandora after removing my headset as well as my Motorola T505 BT to FM modulator out of the picture and listened directly through the phone's speakers.
What would happen is that Pandora would run out of space / memory for downloading the current stream, and as soon as it had a chance to catch up the music would speed up - so it was most noticeable on pitch but if you knew the music enough, you could hear the change in BPM as well (I'm pretty good at discerning minute differences, though I'm not a musician).