I had my A21 paired with the AC12 and it was a synergistic match and I've tried a few against it from Shunyata Delta HC NR , Wireworld "Electra" ,Audio Envy's Ocean and the only one that came close was the Clarity Cable "Vortex". The AC12 is a though cable to beat and very well balanced from top to bottom. If your looking to take your A21 to the next level? Try an HiFi Tuning Supreme fuse and XLR's if your not using them. In my setup the AC12 on the A21 with Morrow Audio MA4 XLR's and SP4 speaker Cables using an HiFi Tuning Supreme or Silver Star (Not made anymore but was an revelation on just how of an amp the A21 was for me)
I still have an Silver Star fuse and Supreme packed away since I no longer have the A21 but the JC5 and needed a different value fuse but the AC12 remains as thus far it's taken down all contenders. Keep us posted on your choice!