Mark, here's my current information about the BlueGiga WT32i board, all contained in comments within my work-in-progress code.
https://create.arduino.cc/editor/tfabris/4c5eea9a-1462-45d7-908d-81a5bb6b0d90/preview

If you're planning to connect one of thse WT32i ports directly to the empeg serial port, and write all the code on the empeg, then all you'd need is:
- RCA cables to the Line In ports of the WT32i.
- 5v power to one particular pin of the WT32i.
- Ground (of course).
- Serial RX and TX in a crossover configuration to the WT32i's RX/TX pins.
- The latter will have to go through the one of those usual RS-232 chips that allows you to connect a UART to an RS-232 cable. I forget what you call that thing. In my case, I'm using an RS-232 shield for the Arduino that has that little chip in it. I expect you'd roll your own there.

I have it paring and working on the following things:
- My Honda car stereo.
- A plantronics mono headset/earpiece.
- A cheap third party stereo bluetooth headset.

With the BlueGiga WT32i development board, the audio quality from the empeg is immaculate. Playing the same song direct-digital from my phone to the car stereo, versus playing the song through the empeg using the line-inputs on that BlueGiga WT32i board (into its ADC and then to the car stereo), the audio quality of those two sources are indistinguishable from each other. I have to strain to hear any differences at all, and then I can't tell which one is objectively "better". Also: No detectable ground loop noise that I could hear (but I don't have my power supply working yet so maybe that'll come later).

It processes play/pause/next/prev commands from the bluetooth devices (ie the car stereo screen or steering wheel, or the pushbuttons on the headsets) and changes the tracks on the empeg as expected. That's enough to get me going, but not quite everything that we want yet...

I haven't been able to get track titles to work yet. In theory the car stereo should be querying my bluetooth chip for the track titles, and then my code is supposed to respond, but it doesn't seem to do that: I don't see the query for the track titles ever showing up on the bluetooth chip's serial port. There must be some configuration tweak that I'm missing.

Sometimes (not always) the car stereo will query for other data unrelated to the track titles. But I can't even seem to get those queries to show up reliably, even when I try. (Right now my code tries to shut off the queries but I'm not even sure about those.)

Their documentation supports it all though:
Command reference: https://www.silabs.com/documents/login/reference-manuals/iWRAP6-API-RM.pdf
AVRCP command reference: https://www.silabs.com/documents/login/application-notes/AN986.pdf
(You have to set up and log in to an account at the silabs web site to get those files.)

From their docs:
Quote:

6.4.2 Examples
Controller asks for three 3 attributes of the current song: 1 (the title), 2 (artist) and 3 (album). Target responses with track details:

Controller (ed: That's the car stereo asking):
AVRCP 0 PDU_GET_ELEMENT_ATTRIBUTES 3 1 2 3

Target (ed: That's me, that's my bluetooth chip supposed to be responding):
AVRCP RSP 3 1 “Some Title” 2 “Some Artist” 3 “Some Album”


The problem is knowing how to configure the chip so that the car stereo even knows to send the queries in the first place. Not sure how to do that, the documentation is extremely hard to follow. Maybe you can look at those docs and see how it's done?

I'm in Ohio for several days and I'll come back to this late next week.
_________________________
Tony Fabris