What made the biggest, most noticeable improvement

My musical fidelity v-dac ii broke and is being repaired under warranty but while it is away it made me think that out of all the improvements I have made to my rig since this craziness started 9 months ago, my dac made the biggest, most noticeable improvement. Every other upgrade made slight improvents here and there but my dac (with upgraded power supply, of course) gave me far better imaging, tighter more defined bass, and more micro detail. I figure the dac in my denon dvd2900 is good and that the dac in the classe ssp 25 pre-amp is good (both sound the same to me) but the musical fidelity v-dac ii with upgraded power supply just blows them away, and I figure it's out classed by many other dacs. Anyhow, besides speaker placement and speakers, what made the biggest improvement in sound quality to your rig including acoustic panels, diffusors and bass traps?