I own the M3 and M21. Both are excellent; the M21 is better in all respects as a sigma-delta DAC and also adds the R2R option. That said, the M3 is a great DAC - you aren't getting three times the performance with three times the price. The network rendering option is superb in each DAC and a valuable add-on that makes both a value option in their respective category as you don't need a separate renderer.