Yes, the I2S lines from the empeg use 3.3V logic, so all good there.

For pairing, there really are no other parameters or options when using Simple Secure Pairing (SSP) protocol. But that protocol has been supplanted in BT 4.x with a "new improved" secure pairing method. I'm suspecting that Harman abandoned the old one, since the normal device that it pairs with is a smartphone, and most all of them have supported 4.x for years now.

One good clue is that the verification PIN it shows on the touchscreen is 8 digits long, not the old-style 4 digits.

I suppose I might have a notebook here with an older BT module inside.. so I could install bluetooth software on it and see if it will pair with the head unit.