Power cable dilemma

I have to ask the community for an explanation to account for an issue I encountered this past week. 
 

I received a Mark Levinson ML-23.5 amp this past week. I plugged it in with a high end power cord I purchased years ago (because it looked better than the cheap cord that came with the unit. Lol). I switched from an ML-9 so I had certain expectation of the sound. After listening for about 30 minutes, I noticed that the amp did not sound very dynamic. I got up to go feel how hot it was and the heat sinks were barely warm. I turned up the volume and listened for another 15-20 minutes. I got up to check the temperature and the heat sinks were still barely warm. Also, I was playing the amp at volume level 28 on my Cambridge 851N. That is pretty high. The sound was still lifeless. I shut everything down and just sat there, dejected. Was the amp defective? Was it just a bad match? Were my expectations too high? I don’t know what made me try it, but I swapped out the power cord with a plain black cord and powered up the system. Unreal. I was now listening at volume level 22 to the same song, with staggering dynamic impact, at what seemed to be a similar volume. Furthermore, after about 20 minutes, I went up to feel the amp and the heat sinks were very toasty! So the question is: what could have possibly been wrong with the original cord that would result in lower volumes and no heat buildup? It is like it was throttling the current. To me, a cord either connects or it doesn’t. It works or it doesn’t. There should not be an in between. Does anyone have an explanation for this?