@soix and @jeffreyw already recommended what I was going to say. I believe your best bang-for-buck upgrade may be a power conditioner.
Like you, I have a more mostest system. I dropped an AudioQuest Niagara 5000 w/ Tornado power cable into the system, and I was rewarded with better everything. Part of me was expecting no change at all, or a slight change only that wouldn’t justify the price. After that result I had, I picked up some AQ Hurricane power cables and also put a Niagara 1200 on my TV system. I was pleased with the results. I’ve played with a handful of sub 1k power conditioners and some sub 1k power cables and haven’t been impressed with the results. Another power conditioner that seems to be getting ubiquitously good results is Puritan, which is under 2k USD.
I would suggest comparing your Monster conditioner against a standard power strip to start. If you’re not hearing a difference, or if it’s just a slight difference, I bet you'll be pleasantly surprised with a dramatic improvement with a Puritan or Niagara.
And, if your results aren’t dramatic like mine were, then sure, put the money into a different component. I can’t speak to your power amp, but I’d think the NAD could be improved upon. My vote would be to consider the Holo Audio Spring or May DAC/pre’s.