In a normal configuration (i.e., you're using the speaker box instead of the empeg), are there any power lines running from the connector to the speaker box? Or does the speaker connect *only* with the 1/8" plug?

Here's what I'm concerned about: Making a speaker cone move, even a relatively small one like the one pictured there, needs *power*.

So if that speaker doesn't have any amplification built-in (i.e., the handsfree kit includes the amplifier rather than the speaker box itself), what I think you might be doing with your configuration is sending a speaker-level powered signal down that 1/8" cable into the empeg's aux-in, which is expecting a line-level unpowered signal.

Do you find that, when you *do* get it all to work (by unplugging after connecting the call) that the empeg's volume has to be turned way down, or that the phone volume has to be turned way down, in order to keep from being distorted and loud?