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.
Degrade in sound over 48 hours. What happened?
Retermination of all connections, complete with Kontakt Electrical cleaner on all terminations and RCA plugs. System is vintage/new combination. Yamaha CR220 15 w/ch receiver 1979. NAD C520 CD player 1999. Bang and Olufsen S35 Type 6311 2-way loudspeakers. AQ Viper RCA 1 meter to CD. AQ CV-4, AQ Type 4 (single run of each, 17.5 feet to speakers). 30" Paradigm Premier metal stands. Monster Gold-Pin connectors (twist type) at receiver. Standard B&O speaker (2 pin) plugs. Result on Saturday was open, very wide soundscape with low lows and clear highs--almost a reverb effect in sound. Monday sound is noticably harsher with muddy bass and distorted highs and loss of quality sound imaging. Only have a basic 6-outlet power strip without any filtering, ect., and all components are non-IEC cables. Any ideas on what caused the sudden loss of fidelity? Seems that the receiver needs more volume than before to get the same sound level a few days ago. It's loud now but not as clear.