What colour is your 6er? I noticed a silver 650i driving through town today!
I got my work done from one of those on Basingstoke Road - John Kleis. To be fair they do seem quite pricey, but they offered me the best deal I could find on fitting the Dension 500 system. If you want to know any more details feel free to ask away!
Just as an update to the thread as a whole - I'm now starting to experience the same crackling (at certain frequencies) and popping (when changing gears, which is very random!). Although the popping seems to have stopped a bit since my 6er was back at the stealers for the brake recall (coincidence??).
My uncle took a look at my original logic 7 amp and said there was some water damage and he cleaned it all up so I'm going to have a go at swapping them back over and seeing if that makes any difference (stealers took a look for me and said I don't have any drainage/hose problems).
Also I'm going to purchase a set of FOCAL 100CA speakers and replace my centre channel speaker (which I suspect is blown) - so if there's anyone interested I can offer the spare speaker at cost price (£30 GBP).