Surprise...a $250 solution is a near-optimal solution
There is so much chatter about upgrading/retrofitting the sound system in stock BMWs, I will offer some suggestions; it may save you a lot of $$ and a lot of time to get to the result you want. In writing this, I am well aware of the conventional wisdom that "stock BMW audio systems are good enough". I am also aware that for cultural reasons, most BMW owners will say "good enough" is definitely not good enough.
There are three basic BMW Audio System Upgrade options *most frequently* considered:
1) Upgrade everything (remove & replace all BMW factory speakers, electronics & wiring)
2) Upgrade just the amplifier to get more power
3) Upgrade the bass end by adding a separate, dedicated sub-woofer amplifier and speaker
Option #1 - Upgrade everything: This is an option to consider if you have a ton of time and money you *want* to spend (either to impress yourself, or to kill time). It is extreme in concept, and often results in loss of some really nice BMW features (like steering-wheel controls and integrated displays). Worse yet, the money-grabbling, shark-like installers will smell the money a mile away and rip-out the *excellent noise cancelling* twisted pair speaker wiring BMW provides, and replace it with some combination of expensive new speaker wires and expensive noise supression circuitry.
Option #2 - Upgrade just the amplifier: This option has a lot of merit in concept, however it turns out that the amplifiers in stock BMWs have almost always been (and certainly true for BMWs in the past decade or so) quite fine in terms of fidelity, and power *except* at the very low end (bass response). Principally, is the problem of too little power in the low end that makes the BMW audio systems just "good enough". Most of us have noted that the BMW Premium Audio adds an amplifier sporting sub woofer outputs, plus sub woofers (usually 2 bass speakers) to the system, but *still* does not have enough amplifier power to bring fidelity to the bass response of the system. Once can certainly replace the BMW factory amplifier (either the standard BMW audio system amp or the Premium Audio amp) with a beefy aftermarket amplifier, however this is typically more expense and hassle than is needed. Let's consider the third option.
Option #3 - Upgrade the bass response by adding a separate sub-woofer amplifier and speaker: This is the optimal solution to bring the BMW audio from "good enough" to an audio system that is "bordering on awesome". BMW owners in general are not looking for glass-breaking bazooka sounds, so the sense of "awesome" is decidedly not "the baddest bass on the block" but rather a full frequency, high-fidelity sound, and spine-tinglingly pleasing across a wide range of program material.
More on adding a separate sub-woofer amplifier and speaker: You will want to consider several key specifications when selecting your subwoofer:
- Has RMS wattage in the 180-220 watt range. Self-contained amp/woofer combination of 200 watts are very common, and 200 watts is fine even for larger cabins
- Has speaker-level signal inputs. Note that the BMW amplifier systems don't have RCA line-level pre-amp outputs.
- Is servo controlled. This is absolutely required to account for changes in the 'enclosure' (which is your trunk) as you may have more/less dead-space in your trunk on any given day.
Granted, for the audiophiles among us, there are many more audio gadgets we'd like to spend money and time on. However for the vast majority of BMW owners for whom "good enough" is not good enough, but "bordering on awesome" *is* good enough, this is your $250 near-optimal solution.