20 year old power center from monster, Circuits still OK?

No.  You can make significant improvements sonically.  AudioQuest Niagara, Core Power Technologies from Underwood HiFi, Audience, Synergistic, Shunyata — any of these and more would probably make an audible improvement in your system’s performance.  Monster is, well, it is what it is and nothing more.  Upgrade to something better would be my advice.  

@soix Agreed . I had actually purchased a couple of those for my HT and an extra system… they seemed to perform like a switch boxes as opposed to a real power conditioner (like I have on my main audio system). In fact it didn’t even protect my processor from a small surge. Then after a few years on of them died… a power conditioner. I called up the company, and they told me it could not be repaired… I called and audio repair shop… they laughed. I forgot what I paid for it… many hundreds… $500? $600?

I always suspected it was a marketing company set to pretend to make audio products and market to mid FI want to be consumers. Their interconnects were no better than cheap store brands… all looks, no performance. But they filled a price niche. I seldom get fooled… “Monster”… what was I thinking?

Monster stuff does work but you can get better, not worth repairing. The thing I didn't like is the cheap case that would vibrate during heavy bass passages. It was audible so I recommend an upgrade.