Bluetooth adapter for aftermarket sound systems

[detch]

Hi,
I have 2003 E85 Z4 that initially came with the most basic audio setup.

I have upgraded my setup with new amp, speakers and retrofitted rear subs. And this showed that my bluetooth dongle isn't that high quality. The sound is flat and I need to crank the bass/trebles pretty high. And even the results are pretty muddy :/
So I have contacted the producer and what I've learned is:

Good morning. In the BMW E83 / BMW E85 models, the situation is as follows:

The adapter is built in accordance with BMW technical specifications, and while it works with standard head units, in the HiFi system (with additional subwoofers under the seats) the sound becomes noticeably flat.

That’s why we offer two types of adapters: Standard and HiFi. The HiFi adapter has a wider frequency range and performs much better, even though it does not comply with BMW’s official specifications.

This would likely solve your problem, but there is one catch. The HiFi adapter may not be detected by a standard head unit, meaning the AUX option might not appear in the radio.

I’m not certain how it would behave with a custom setup.

This certainly makes sense.
But what can I do then? Any tips?