Basic upgrade: $499.97
Head unit + 2 pairs of speakers.
You are looking to improve all aspects of your system. You want mp3/wma playback, decent bass, cleaner highs, and louder overall listening levels.
Alpine CDA-9856 CD/MP3/WMA Receiver $199,99
The CDA-9856 is a great starting point for a system at a great price. You get an internal 4x18w amplifier, CDRW/MP3/WMA playback, 3 sets of pre-outs for future amplifier upgrades, and an AiNet connection for future changer, ipod-control, sat-radio, or hd-radio upgrades.
Alpine Type-R SPR-57LP 5" x 7" 2-way Car Speakers $149.99
These speakers are an easy drop-in to Mazda3s and Proteges with 5x7 front speakers. At 40w RMS power handling they are a good match to the CDA-9856 receiver.
Alpine Type-R SPR-17LP 6-3/4" 2-way Speakers $149.99
To compliment your front speakers a pair of 6-3/4" rear speakers will give you some additional bass. Mazda3 owners may want to simply get a second pair of SPR-57LPs to fit the rear 5x7 openings.
Wiring Harnesses, installation kit, and instructions: FREE from Crutchfield.
BASS Upgrade: $459.95
Amplifier + 2 12" subwoofers
You think the factory system sounds fine except there is no bass and you need something pounding out bass lines in your hatch or trunk.
Alpine MRP-M450 250W x 1 Car Amplifier $199.99
When thinking about bass, you need to pick an amplifier first because it's the biggest ticket item and it will determine which subwoofers you can run. This Alpine monoblock is priced just right to fit our budget and will put out 1x400w @ 2ohms. 400w is more than enough power to compliment the factory speakers. Any more and you'll be drowing out the midbass and treble.
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Alpine Type-E SWE-1242 12" 4-ohm subwoofers $149.98/ea
No bass system is complete without a pair of 12" subwoofers. Sure you could find a single 12" subwoofer that can handle 400w but these babies will give you the wow factor when you open your hatch or trunk.
Q-Logic QLH-12D125 Dual 12" Hatchback Subwoofer Box $69.99
A basic dual subwoofer box will be fine for the subs we picked out.
StreetWires PS08BI 8 gauge<!-- Ext Adj Item Price --> Amp Wiring Kit Blue with Patch Cable $39.99
Since you are only connecting one amp rated under 500w an 8 gauge wiring kit should be fine.
The Alpine amplifier has speaker-level inputs so you won't need a line-out-converter. Add on $10 for some 14-18ga speaker wire. Shipping is free from Crutchfield.
SQ Upgrade: $489.95
Amplifier, speakers, and 10" subwoofer
You like bass...but more importantly you like a well rounded system with great SQ. Eventually you will upgrade the head unit but for now you can live with it.
Alpine MRP-F250 40W x 4 Car Amplifier $199.99
Start with a moderately priced 4 channel amplifier that can be used to power both a set of speakers and a subwoofer. This amplifier also has speaker-level inputs so you won't need to purchase a $30 line-out-converter.
Infinity Kappa 682.7cf 6"x8" 2-way Speakers $149.99
You could splurge and buy a set of $200 component speakers but these speakers are 2-ohms and will give you a little more power from the alpine amp. They will see 2x50w instead of 2x40w. It's a small difference but every little bit counts when you are on a budget.
Infinity Reference 1040w 10" 4-ohm Subwoofer $59.99
This infinity sub is priced cheap and will go nicely with the 6x8 speakers we selected for the front. It can handle more power than the amplifier will dish out but it will make the most out of the 100w with a 91db sensitivity.
StreetWires PS10BI 10 gauge Amp Wiring Kit $29.99
You'll be running under 300w so a 10 gauge wiring kit should be fine. If you plan on upgrading the amp later you might want to splurge for an 8ga kit.
Add an additional $10 for speaker wire and you're good to go. Shipping is free from Crutchfield. When installing the equipment, you can opt to keep your factory front tweeters and rear speakers connected to the stock head unit. I don't recommend it but the option is there.