Hi Lippo, owning the S370, a couple of rungs down the Sony Blu-Ray player ladder, I think I'm comfortable in recommending the player as is, the onboard DAC already being really quite impressive for both music and movies. It doesn't really give a whole lot away to my £600 Marantz, but, for an offboard DAC, I'd also be quite comfortable in recommending my Spitfire II. Also very close to my Marantz (in fact, there's very little difference, which shows how close most DAC chips are to each other these days).
Some will also recommend the Cambridge DACMagic, but, having tried that myself for a couple of weeks, I found while it led in connectivity terms (brilliant), the Spitfire II leaves it standing where it counts with music, simply by being more musical and lifelike. The DM I found rather sterile - all the detail, but none of the soul. Bit of a deal breaker for me that one, so it went back to Richer Sounds. The Spitfire II is £250, has one each of Coax, USB and Optical and all are good, although the USB is limited to 16-bit replay, the others upsampling to 24/192. If you wanted the full monty for USB, check out Emotiva's XDA-1, which is currently an unsung hero by all accounts. £349 from Item Audio and you can get it for two or three weeks home demo. Either of these two should do you quite nicely.