My experience with digital sources are a bit different overall. It may very well have to do with the Pass equipment not getting very much in the way. In the past, I had tested out Emotiva's XDA-1 as well as the Teac UD-H01. At the time, computer audio still seemed to be quite early on and I just wanted the Mac Mini to have a balanced connection that didn't sound as poorly as it had. So while both DACs had been an improvement over the Mini analog, both fell short of the Marantz when it fed them via SPDIF.
I came to the conclusion the most of the lower cost options were a step up from integrated, but few would give the Marantz a real run. In my system, the various digital sources all have notable differences that are immediately apparent. This morning I could very clearly hear the advantage the M51 has on percussion over the SA-11S1. On the SA-11S1, they were not presented with the same weight and definition and thus not giving it the same body. Both are highly resolving, but the M51 has a bit more image depth and allows every instrument to be more clearly defined. The bass is not just stronger, its more defined and is adding a nice fullness and richness of sound without any loss of definition.
Other DAC's I have considered was the PS Audio PWD (now in Mark II edition), Bel Canto Series, and others. The M51 was the first unit to provide a solution for digital volume control with very little to nearly no loss and quite a new approach. I can say that the sound is very impressive and is a clear step up from the SA-11S1. Would any of the others go further? I don't know and I would sure like to find out. But with the new PS Audio costing $4,000 list and my cost on the M51 being less then half that, I have doubts it would provide a gain with the cost. In some respects, the PS Audio would be inferior as it provides a digital volume control that doesn't match the solution in the M51. This volume control issue is also true of the Bel Canto.