Wait, what? I thought this thread was about changing your original head unit?
Are you just looking to change the speakers and keep the Concert head unit?
The Concert has amp outputs for front and rear so you don't need a line-level converter. Sound quality is pants with those anyway. I'm sure I've seen an adapter to get phono connectors from the Concert so you can drive an amp directly - I'll have a look.
Edit - found one:
https://www.ebay.co.uk/itm/Adapter-C...I/283313111847