First, remember that the compression method and bit rate of the music being played has the most effect on the sound quality. FLAC encoded music will always sound the best, with the trade off of extra space consumption on the DMS cartridge.
I run a KHD-CX910 and I had a KHD-C710 for a short time. In my experience, the KHD-CX910 sounded better. I think this is due to its 24 bit DAC as mentioned earlier. The KHD-CX910 was from Kenwood's Excelon line, which is their high end equipment. KHD-CX910 is to the KHD-C710 as Lexus is to Toyota. And they do both have RCA jacks.
Neither of these units came with USB 2.0 cradles originally. The USB 2.0 cradle has to be purchased separately, or could obtained from a VW or Audi Phatbox kit. I got my USB 2.0 cradle from a VW Phatbox kit, which used to be available for about $125 shipped, but they are all gone now and only available from EBay auctions. The VW phatbox is roughly equivalent to the KHD-C710, so that one is for the wife's car
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