Degrade in sound over 48 hours. What happened?

Based on your description it seems that you are experiencing the effect of worse power fed to your components i.e. more use thus more noise power you get and voltage may sag.
If you have acces to a volt meter you can check (if you don't know how to do it better have someone else do it so you can learn how to, SAFETY FIRST!) if voltage at your home is lower during weekdays vs weekend it might be the cause of it.
Will have to search the archives on power conditioning and read plenty about it.
Did it happened from last weekend to this monday?

Regards
Luis

Might be fluctuations in the incoming electric service.
Might want to check (clean) the grounding connection for your service.
Weather conditions (humidity) seem to play a part on this,
assuming your equipment didn't develope any problems.

I would suggest your system is in the process of re-break-in due to retermination. It will take a week or two.

If you do use a volt meter to check you line voltage, remember that the SMALL slot is HOT, and the BIG slot is NEUTRAL (ground). ALWAYS insert your neutral (black) probe first!

Substitute, if you can, another IA and cdp without the power strip into your system and see if the problem is duplicated to confirm a power problem. But I don't think it's an A/C issue...

It's very possible it's the receiver (I'm assuming you reterminated with correct polarity). Could be worn out power supply capacitors, especially after 25 years. Other possibilities are damaged cable terminations or a problem with the input/output jacks on the receiver. See if lightly tapping or pulling on the input and output cables near the back connections result in a popping sound. Or try tapping on the volume knob and see if there's static/grunge.

Use headphones on tuner. Then CD. This will rule out interconnect problems. Twist-on connectors are not 100%, the HP test will bypass the speaker cabling. If both speakers exhibit the same problem, see Gs's answer above. 25 is old in electronic years.

I thank you all for the advice. Not sure what might be the culprit. I mainly listen to CD's so this pathway is the one I am concerned with. The radio and the record player are unchanged. However, all CD's that sounded fine on Sunday sound tinny and distorted today as the low end just disappeared. For a remedy, I like the idea of trying a different NAD C520 CD player (I have another one) to compare CD sound output. Then, if no change in output, I can try another new amplifier in the mix. Overall, my terminations in cables and speaker cable appear to be tight and clean. Plus, the power appeared clean on Sunday with a very dark background and an extended bass as a result. A few years ago I tried a Wireworld Electrifier power strip and an Audioquest AC15 IEC power cable with this system. There was no audible difference to the old power strip or to plugging everything into the wall outlets. Can the clean power I had just die out that fast without some component failing? As far as for reburning in the cables and speaker wires, they have been used almost daily for five years and more often than not the receiver and the CD player remain on 24 hrs a day. Audioquest said it takes about 60 hours of play to burn in new cables or IC's (not active for over two weeks is like new). Again, thanks for the great suggestions--something should work here. If not I will upgrade all components and retire the vintage 1970's pieces as 25 years is a long time indeed for anything to last! A new pair of Paradigm Monitor 3 v.3 speakers and a Marantz PM7200 Class A amplifier are ready for "the next phase" if needed.