Both seem to be nice decks, the 745 also has the 4 volt preouts, as opposed to the 1.8 volt ones on the 645. The AUX in is a nice feature also. Are you going to be running multiple amps, if so I would say go with the 745. As far as the D/A convertor is concerned, I would think that the 745 would sound cleaner, remember the 9255 deck they made back in the day with the Burr Brown D/A convertor, it was a very clean deck. Hope this helps a bit.