Bluetooth Headset Sound Quality

This may be a long shot but has anybody looked (listened) to a variety of
handsets using a known quantity wireless Bluetooth headset?

My hearing isn't perfect and I'm struggling with a few handsets and a
Bluetooth headset.

The reason why I ask is that at the beginning of the year I was using a Sony
Ericsson T610 with a Sony Ericsson HBH-662 headset. Great sound quality on
the headset and I was happy.

Then I changed handset to a Motorola V635. Newer, more modern handset, more
functionality blah blah blah but indistinctive sound from the headset. Way
too much bass or not enough mid and treble, meaning it's almost impossible
for me to use with the headset in a quiet room, let alone at the wheel
(say).

I try a Nokia 6230. Great sound quality and I'm very happy, but somebody
gives me a BlackBerry 8700c from the 'States. So I try this... poor sound
quality, just like the Motorola - no, actually, worse heh. I've tried a
couple of RAZRs that were better than the V635 but not as good as the 6230.
The 6230i didn't sound great, either...

I could go on... but I won't. Is the reason for the differential in sound
quality associated with the signal strength, the bitrate or that the
manufacturers only send certain frequencies to the headset for certain
devices?

--
The DervMan
www.dervman.com