hmm.. im not to sure, because i have 2 amps, and all original FRONT speakers, along with a 10 inch sub, and a 15 inch PA speaker..
(take a look at the pictures)
ANYWAY.. you need to understand the general concept of speakers. the LOWER the watts the LOUDER they will be is the basic rule of thumb.. now that u all think i am nuts, let me explain.. the stock BMW speakers are LOUD because they only NEED 25 watts to power them. Where as aftermarket ones could be inefficient and could use over 50 - 100 watts. That is where the idea of an amp comes into play. If you have 2 stock speakers at 25 watts each, then your radio can handle them without a problem, but when your speakers DEMAND 100 watts, and your radio can only put out 25 watts, you must require the use of an AMP to help you out.
Watts only tells you how much power your speakers can take before they blow up.
you MUST look at the DB rating. The DB rating tells you how loud your speakers will be. DB is really how loud a speaker is at 1 watt.. the higher the DB number, the louder the speaker..
if you are looking for GOOD speakers to replace your stock ones without going over budget, the answer is simple.. keep your old speakes, or look for ones that have VERY HIGH DB with lower watts. BMW has seem to have found the correct ones for the amp already installed in the car..
I have kept all my original speakers, and I have spent well over $1500 on my sound system in my car.. (closer to $2000)
your OHMS is also messed up.. your stock ones where 8 OHMS because you probally have 2 side speakers on your door.. they are all connected together, and if you put a 4 ohm speaker into your system, it will be alot harder to hear. MORE power is required to power subs with LOWER ohms.. think of it like this.. the higher the ohms, the less power.. but your radio and amps can only handle certain ranges... if you keep the stock 8 ohm speaker in your car, the total ohms combined with the other 2 speakers will probally push your ohms down to 4 ohms per channel, which is what u want. if you stick a 4 ohm speaker in your kick, it will cause the total ohms to drop to around 2 - 3 ohms (i didnt do the math) but it may cause your radio to be unstable. amps can handle this normall, but radios sometimes cant, and u can wreck your radio (the lower the ohms, the harder it is on the radio or amp. lower ohms is almost liek a short circuit. 0 ohms is a dead short.. . so u get the idea.) if the ohms are two high, the radio can not handle it, and will blow the speaker apart by pumping to much power into it and melting the inside of it..
now.. what is the problem? You want your system louder? You have to get an AMP, you can get a one and mount it on the back of your rear seats like I have, they look nice there.. do a nice job wiring it up and it will look fine.. if you need lower bass, you will need a sub. SMALL component speakers like you bought are not normally made for bass .. they make a few that are small and PURE BASS but not much in-between that will out perform the stock ones.. you have to look at single speakers as well, anything that is 2 or more components is PURE JUNK and sound horrible… even the high quality ones..(they lose all bass) if your BMW is like mine, it should have 2 speakers in your doors as well, these are only high pitch tweeters.. and combined with the midrange in your kick, it makes a perfect harmony.. if you want to add bass without a sub, you need to look at your rear speakers, or a sub.. you said you don’t want a sub, so you need to replace the rear speakers with 1 component speakers (you can even keep the stock ones if you want) and you need to get CROSSOVERS to lower the sound to more of a pure bass.. the only thing that will effectively work is an amp, but a subwoofer crossover will change the speaker to a more basically speaker, and should be what u are looking for..
basically, this rule is important when sizing up small speaker like in your car. When you have a large speaker or a sub, most people don’t care about the DB and ONLY look at watts.. well.. that’s nice and all, but u can really push your system to the limits if you know the DB rating of a speaker. Lets say you cant chose between 2 subwoofers, 1 is 250 watts and 100 DB, and one is 1000 watts at 88 DB .. and we should assume that both have the same basic build.. 10 inches, 1 inch rubber around it. Now.. most people will say , HOLY S*** 1000 watts is MUCH better, then that crappy 250 watt sub. But let me assure you, the 250 watt, 100DB sub that is in my car is just as loud as that 1000 watt sub..
if you need to look at an amp to power it, would you rather buy a 100 – 200 watt amp to power the 250watt speaker, or would you rather buy a 500 – 1000 watt sub to power a 1000 watt sub? Well, it might not matter, but PRICE will be play a big role. Now.. there are 3 types of amps.. Cheap with low watts, middle priced with HIGH watts, and expensive with LOW watts. It’s all about how efficient they are. Now a 1000 watt amp might seem nice at first, but you must then buy LARGE wire and a LARGE CAP to handle it. It also will cause damage to your alternator and sometimes computer systems if not installed right.. where as, if you get a amp similar to mine, which is only 35 watts at 4 channels and a 75 watt and 2 channels, (both about 150 watts total) with a much larger price tag, not only are they cheap to install, but will last MUCH longer, as they do not produce a lot of damaging heat. They are amazing just as loud as those 1000 watt amps, but are made for normal or high end/low powered speakers.
my 250 watt speaker that is hooked up to the 150 watt amp is louder then any other 10 – 12 inch speaker that has come my way. It also cost more.. haha.. and the same with the 35 watt 4 channel amp, which I use for my entire cars speakers..
i would reccument what i did to fix yoru problem.. get a high end amp.. i LOVE CROSSFIRE AMPS, and you can getthem on ebay alittle cheaper, weather used or new.. they coem with a high price tag, but they sound much better, and liek i said, after you figure in wire size and teh use of a CAP, the more powerful ones are ALWAYS more expensive.. you can use cheap 12 AWG wire with tehse amps, where are with a 1000 watt amp, you will need 0/2 wire, which is normally a few $ per foot, and also a CAP, which can run about $100 new.. so .. lower watt are always better..
get 2 crossovers if you dont already have them, and use them to configure your speakers.. the ones in your kick set up as the Woofers, and the ones in your door, setup as teh tweers with around -3 DB. then for the rear speakers, use the rear output for the amp.. it will be the best investment u have made.. better then the radio itself =] ..
component or stock? 
Linear Mode
