One of the continual features of R&D with audio equipment is the constant reminder about power supply quality. As you improve the core design and it becomes more transparent, you always hear in your listening tests how it becomes an open window on the quality of the power supplies. You then make further improvements on the power supplies and the performance lifts again. This iterative process is a healthy indication that you're making great progress. It's often then only a matter of the increasing costs going into your designs (with say more layers of regulation and better quality capacitors) that determine when you freeze the design – a nice problem to have in a way. The only situation that ever worries me is when I can't hear the changes we make!