I want to replace my factory stereo with a stereo deck that has USB/AUX input. But I don't want to spend as much, if I buy a stereo deck at Walmart/Best Buy will it affect my sound quality?

Also, I would love to upgrade my four 6x8 factory speakers, add a sub-woofer, and add an amp. Any suggestions on what to buy?

Please and thanks, will choose top answer