Hi
I tested the M51 (no preamp enabled) at home a couple of months ago. It was very good, very deep meaty bass and not a single hint of harsness in the treeble. However, when I compared it to the MDAC and I could not justify the price difference: they sounded very similar with my amp/speakers and yes, there where subtle differences in the presentation, but the MDAC was just too close at half the price, being also a digital pre-amp. And when I say close, I don't mean that the MDAC was the one behind at all.
In the end I got the Mdac. The HDMI input is a nice feature but you have to make sure that your bluray converts the signal to LPCM over HDMI (many brands do this with their latest models: Sony, Oppo, Denon...).The M51 CANNOT process DSD diretly (for example the digital format present in SACD).
If I remember correctly, an adapter from HDMI to optical/SPDIF only costs 40 pounds on amazon.
I hope this helps.