One of the biggest diferences you are going to experience is the size, quality and design of the speakers. Most car stereo speakers are single cone without a tweater section or a decent crossover. Aside from that, they are generally much less responsive and the enclosures are not designed for sound quality. Because the speakers are so much better in the home (even the cheap ones) and they provide most of the coloration in a digital system, you will hear a big improvement in SQ.
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Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner

TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv


Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner