Great to read your update.
I like your idea about the equalizer-amp and a bluetooth module. I have modified a Blaupaunkt radio with a built-in module that allows a signal pass through when not playing. Getting decent sound quality in the Murena is a bit challenging, one issue being that even with bigger speakers, the rear is essentially open which means that the low end "short circuits" back to the rear of the speaker. I think the door speakers will work much better in this respect, but I don't have them, and like you, I prefer not to. I've worked on making a very compact "subwoofer" that fits above the glove compartment...
Rear reflectors are available from Simon, but could you post a picture of exactly what did not pass?

