Speak to Lee at Platinum Car in Coventry. You can say that you know Sam, the one who bought the Hertz HDP4 off him few months back! He'll know me, hopefully! He is by far one of the most nicest, neutral, 'unsalesy' guy I know - genuinely passionate about car audio. He gave me endless advice and made nothing from it.... I didn't go to him since he's too far for me to go to but had I been living in those neck of the woods, I'd be all over his shop floor like a rash. He is really knowledgeable in acoustics / sound and seems to knows his stuff very well!
Just give him a call mate and speak to him, tell him what you have and what you're after and if anything, you'll only walk way gaining valueable knowledge
Sam.