Absent an (active) preamp one is effectively listening to the source device's output stage, which is most cases has been designed to drive a preamp and not a power amp. In addition, many digital volume controls lose resolution at lower signal levels. In your specific case, it's relevant that the Chord equipment has both a very well designed output stage and a very high quality digital volume control implementation. I'm not surprised that the setup sounds better than with the Marantz in circuit. Not only are you eliminating the electronics, but a set of interconnects. To echo previous posts, given well designed source electronics, no preamp will most likely be better than a poor one. Lastly, preamps generaly do, of course, provide source switching which many people need - statement of the obvious perhaps, but relevant nonetheless.
The Best Preamp is no Preamp?
So recently I've discovered the possibility of completely removing my preamp from my rig. I've never heard or considered this before, so much audio tradition... But in going directly from DAC to amplifier the sound quality is absolutely incredible, instantly had me grinning. Using music server to Chord M Scaler to Chord Qutest (cut out Marantz SR5015) to go directly to dual Emotiva XPA-DR1 monoblocks, to GR Research's 24 strand speaker wire to Magnepan 1.7i's. Only difference is running volume on server vs Marantz remote, sound quality is the biggest jump I've ever heard with any gear.
Have you guys had experience cutting out the preamp from your rig? What's your thoughts?