I had a thought. Been thinking about adding a subwoofer at some point to the factory system. Rather than using high level inputs on a sub amp, would it be possible to add à feature incorporating perhaps RCA outputs for use with connecting a sub amp.
Just some thoughts I had and I'm not at all sure how feasible the idea might be.
I'd expect this approach to have some disadvantages, mostly that the subwoofer only works for BlueSaab output, not when playing radio or other IHU source.
Also, the output from the RN52 module is not line-level, and it's balanced. You'd still need some amp and other circuitry to deliver it to the sub amp in the most popular format. At that point, you might as well use a more conventional after market sub-amp driver approach from the existing IHU, or, depending on your IHU, there is an output for amplifiers (e.g. AS3 on the 9-3 has a separate amp for the door speakers (woofers). It is a proprietary plug but you could probably get the required cable for splicing from an auto recycler.
The last approach is how I added a subwoofer to my 9-3 several years ago. I did not have the separate amp for door speakers installed in my 9-3 so the amplifier connector was available. On my 1999 9-3 it is a standard DIN plug, so I just got a standard DIN socket and connected wires in a piece of Cat 5 cable to the pins of that socket and ran the cable to the hatch area. I bought a powered subwoofer that had line level inputs for the left and right channels and a remote turn on connection. That is exactly what the amplifier connection provides so I just connected each wire from the amplifier connection to the corresponding subwoofer connection - dead simple. I also had to supply power from an unused connection in the fuse box. Been running great for years.