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I want to replace the BOSE sub in my B5, RS4 Does anybody know where I can find a 13cm sub had a look around and everyone seems to only start with 10" units. I really dont want to put in a box with an enclosed sub in the boot.

5" would be hard to come by. Though what I would suggest is maybe a component speaker set and just use the main driver from that? The main thing you are after is lower frequency response. A lot of "standard" speakers (ie not subs) dont go that low.
However these - http://www.caraudiodirect.co.uk/pioneer ... -6555.html have a lower range of 35hz which is on par with a lot of smaller subwoofers, most other speakers start around 70hz and higher.
All you need to do is get the crossover right and you should be good to go.

thanks bobbler. you seem to know your stuff regarding car audio! couple of q's for you; would it be easier if i just changed the speakers in the rear doors for something a bit better - in that way they could help the original sub out on low frequency? regarding crossover on the pioneer what do u have to do? essentially the bose is lacking a decent low frequency - im changing the headunit which should assist matters. but i really want to try avioding sticking a sub in the boot.

I know a little, mainly from having always had some form of modified audio in my car since I started driving, secondly because Sextons installed one subwoofer for me by drilling through the brake lines on my old Mazda - its at that point you realise that you can do it better than the "pros" LOL. My issue is that I am no good at woodwork and design stuff, but technical I am fine with. So it will sound nice but look like arse LOL.

The rears - I probably wouldn't bother personally, I would think a decent set of fronts + a sub should be more than ample. It will certainly be streets ahead of the Bose setup that's for certain when its all setup and running great. Spend more on the fronts in preference to a set of rears which will be somewhat wasted. Unless you go for midbass driver - but watch the depth, they are usually quite deep mounting depths for a door

Crossover - this is (in laymens terms) a device which sends the right "range" of sound to the correct speaker. So for instance you are wanting to send bass to the subwoofer, mid to the front main drivers, treble to the tweeters. Each of the speaker types is designed to excel at producing just a portion of the overall sound range while at the same time not being very good at the other notes. You wouldn't want bass type notes going to the tweets as they just wont be able to make that sort of noise. Same vice versa - subs are not very good at hitting high notes that a tweeter can do

So the sub range is something like 20-5000hz, the mids 70hz up, the tweets handle the upper bits up to 20Khz (covers the 20hz to 20khz that we are supposed to be able hear). The setting of the crossover will allow you to have a smooth transition of sound from each driver to the next when you play a sweeping range sound, so the sub starts making its noise as the mids are stopping making theirs, the tweets start making theirs as the upper range of the mid is being reached.

A set of component speakers (mids and tweets like you have in the front) will usually come with a fixed crossover - the little box in the left of this set picture for instance - http://www.caraudiodirect.co.uk/alpine- ... -5452.html Because the speaker maker knows the range of both the drivers they can give you a preset unit which will do the right sound range to each bit.

Crossovers can be found in quite a variety of types. So you have the fixed box as above, adjustable crossovers on most amplifiers (with the ability to set subwoofer ranges and mids/tweets ranges), standalone crossovers (Audiocontrol make several of these) and even some headunits have them too - my Alpine 9855R has a three way crossover built right in so you can do all your tuning from up front where you are listening.

Have a read of http://www.bcae1.com/ if you want to know just about anything car audio related from a technical standpoint.

Sub - personally speaking I would go for a sub in the boot. Reason being is that a sub performs best in the correctly sized box. Now looking at the information posted on your earlier thread, the Bose box is 10.2 litres in size, so you are going to want a driver which has a very similar recommended box volume to get the best out of it.
Now this is where you see the problem. My sub (which I have yet to get round to installing LOL) is an RE8 and that has a recommended volume of 0.35 cubic feet - or just shy of 10 litres and that's an 8" sub. So finding a 5" driver (the 13cm of the mounting hole on the existing box) that is going to have an approximate requirement of 10 litres is going to be very much hit or miss.
I didn't know the size of the existing box at the time so apologies for the suggestion of the 5" from a speaker set.
The other thing with that Bose box (having checked mine) is its some awful nasty plastic which is also not going to be great for sound.

Options:
1. Take out the existing box and a build one in its place from MDF/Ply or Fibreglass. This gives you a greater range of sub woofer choices in the 8" size as they will all want around 0.35 cu feet. It will sound lovely IMO. But its a lot more work than what you have in mind.

2. Build a box in the wheel enclosure - this is how I am doing mine, I have no spare so I am not missing out on anything really, plus I get to keep my boot space and the installation is fairly stealthy too as my amp will be able to go in that same space.

3. Another option entirely - low profile pre-built amplified sub under the front seat : http://www.caraudiodirect.co.uk/pioneer ... -6820.html
Or similar

Bobbler - thanks for the post really appreciate the effort and time taken by you to explain things. Are there any decent audio-specialists/installers that you would recommend?

Hi mate, best bet would be to use someone local to you. Maybe have a search of Talkaudio.co.uk to see if there are any recommended guys in your area.

Thanks.

hey bobbler, spoke to a few installers and the plan is to either make a custom enclosure to sit flush under the glovebox or remove all the trim from the rear quarter and build something custom again for the sub and new amp auidson. looking at a 5 channel amp to power the sub and speakers. and either a jl audio or hertz sub. what you reckon? been quoted £700-ish for the whole job...........

Sounds good mate to me, did you get to hear any of it at all and is that price including the parts too? That would seem pretty good for the money.

Yep price is for the complete job. However, I didn't get to hear anything - but before I make a decision will be visiting 3 more installers and see what they are suggesting.