Edit: apologies for the long post, treat yourself to a beer if you manage to read through it all!
Ouch, sorry to hear of the pain!
Needless to say the aftermarket industry has you covered!
The way I see it, there are a few different ways of getting around the issue of the carver amplifier but the fact that it accepts a digital signal does complicate things.
I'm going to assume that you don't have nav (makes things easier in my opinion).
Option 1:
The easiest way would be to get a non-dsp head unit to allow for an analogue signal into an amp. The only issue you'll face is having enough output channels (as you mentioned, there are no passive crossovers). For simplicity's sake, you'll need 8 output channels. (tweeters x2, mids x2, woofers x2, subs x2)
You can use the crossover's built into most amplifiers these days.
Pros: probably the most simple option
Cons: you'll be left with minimal eq configurations
Option 2
If you're able to stretch your budget, you could incorporate an aftermarket audio processor into option 1. This will give you a plethora of additional tuning options. However the issue here is finding a processor with enough outputs. To get around this you could (for example take an audison bit10) which offers 5 output channels. 1+2 will output signal for both the tweeters and the midrange drivers leaving 3+4 for the woofers in the kick panels and then running the subwoofers in single channel on 5. The amplifier built in crossover will allow you to separate the signal from the mid and tweeter. Alternatively you could run a passive crossover in between the amplifier and the drivers (though you will be susceptible to power wastage through a passive crossover)
Option 3
I think this may be considerably out of budget, but for the sake of helping anyone else who reads this, it may be helpful.
Mobridge makes a unit which accepts the bmw digital signal and then outputs an optical signal in the form of a toslink connector. From here you have a signal to do with as you please (either optical into a digital sound processor or a digital-to-analogue adaptor and then onto the amplifiers)
Option 4
You could look into your water damaged unit being repaired.
Gordon, aka the amp doctor only has good things spoken about him. So it may be worth contacting him to see if he can help.
http://www.ampdoctor.co.uk/about-us/
Amplifiers
Space in the z4 is at a premium. Full range class D amps have come a long way in the past few years so you stand to benefit from better thermal efficiency. JBL, JL Audio, and Rockford Fostage (to name a few) make some excellent class D amps that are incredibly compact.
Personally, I'm a fan of a nice simple setup, pair of tweeters, pair of woofers on the front stage. Three way front stages are perceived for being difficult to sound 'right'. Though I've never has any experience in this department as I opted for a 2 way in my setup.
Subwoofers
I've also read that the carver subwoofers are of an odd impedance. Aside from changing them out, you can adjust how they're wired (i.e. loose the stereo element) to either raise or lower the resting impedance. Also, it is the resting impedance which ratings go by, but once the driver is moving, that impedance does change (leading some to argue that it's somewhat irrelevant to get caught up on resting impedances of a driver)
Aftermarket solutions, well JL Audio offer a 6w3 which seems to be favoured by some owners on the forum. Though not cheap, it certainly offers a lot for it's size.
As for the main pitfalls, my biggest hurdle was where I mounted the amplifier. Initially I wanted it to be behind the seat hidden away, but later opted for it to be mounted on the back wall to keep the power cable as short as possible.
Other options
You could run a parrot (or similar) bluetooth car kit. These have a built in amp that is fed to your speakers. Given the nature of the digital system these would sit between the amplifier and the speakers. Probably the cheapest option from a financial perspective though you'll need passive crossovers and it will more than likely sound awful.
Given the money that is tied up in the other BMW amplifiers, you may be able to sell them to allow for new front speakers and subwoofers. Couple this with option 2 and you'll be on your way to having a system that not only sounds great but can go nice and loud! As for speaker choices, I went with a set of Hertz HSK165. Main reason was they fit the budget nicely and they sound great (these are being run actively, i.e. each component is wired to a separate channel). Audison also offer a similar product, with a different badge. Perks of both companies being owned by the same parent company.
I've found that JL make fantastic subwoofers and amplifiers (slightly biased opinion here, and yeah I'm a fanboy!) but their speakers, for the money, are overpriced and overhyped. Having heard the c2's and c3's they are both incredibly sharp and ultimately ruins the listening experience. The c5's as fantastic as they are...are stupidly priced (in my opinion).
Focal are another company to offer fantastic speakers, so certainly worth considering them.
A final note, do avoid fli, edge, vibe, inphase. Those products sell in great volume due to a higher marketing budget than product development. Certainly not worth your time given you're coming from a carver system.
I'm sure I've probably missed something, but please feel free to pm me or reply here if you have any other questions!
Hopefully it's of some help!
