My son has a Mazda 3, Clarion CX501, $189.00 from Sonic, has 6x8 stock speakers, and just added an inexpensive Orion 10" entry sub $65.00 (sealed enclosure) and a Phoenix Gold 2 Channel used amp $20 bucks bridged to 160 watts x 1 @ 4 ohms.
Clarion CX501 (CX-501) In-Dash Car Stereo w/ Bluetooth...