I retained my factory head and bypassed the h/k amp completely, then wired in a pair of components up front and a 12" sub in the trunk. No rear fill channels for me.

To achieve good midbass up front I had to fashion a baffle and cut out the door shell behind the mid-bass driver. The speakers I purchased thrive on airspace, so I had to do a lot of work to provide that. Once I did though, ooh baby, gobs of midbass up front and excellent blend with the sub in the trunk. Read this paragraph as... you cannot just bolt in a high-end set of speakers using factory mounting method and get awesome results. How the h/k gets as much bass as it does up front is still a mystery to me. I think they play serious EQ tricks to do it.

I have yet to fully tune the system as I'm still undecided about tweeter position in my coupe. I have them currently in the sail panel behind the side-view mirrors, but will work on an a-pillar solution to try out next. That will raise the soundstage up and forward. The stock location on the coupe is terrible... soundstage is between your chest and the dashboard. Weaksauce!

The tutorial you posted originally is a good way to go for a budget improvement. I have read good things about BSW and every debate that gets started evolves into a "yeah, but we did all the research and development!" to explain the big price tag. I have never heard a car with that solution in it though. When I was originally researching an upgrade I was turned off by them because back then they were just rebranding MB Quart and I was not a fan of that brand at all.

I had a/d/s/ in my E39 that I transferred into this car and it never quite did the trick for me in the E46. I suspect it was part install and part tired components as I had owned them for 10 years and they were in 3 different cars. I went with Hybrid Audio Clarus components and an IDQ sub - very happy with these choices now that I've worked out the install. I think you get more out of the install than the components you select cuz an expensive driver poorly installed will sound bad too.