"James Harris" wrote

My hi-fi sounds great sometimes and not others. I am
told that it needs an hour's warm-up and this could fit
with the times it has sounded good. For example, great
one evening but poor the next morning. I leave the
electronics on but not playing.

Perhaps you might consider a power line conditioner.
These devices reduce RF, EMI and other distortions
that leak into your hi-fi gear from AC power lines.
While some equipment does not appear (sound
wise) to benefit from the device most will, IME.

Most audio stores will allow you to demo these
devices for free so there is no financial risk. The cost
of these units start at $200 and run up to $4,000
depending on your needs. For a periodical review of
these units check out Stereophile issue Vol. 17 No.
12.


Is there a CD that can be played to loosen the
system up? I suspect the speakers most. Bass
extension is particularly absent at times. Any
suggestions as to whether playing a very low
frequency sine wave or similar could do the trick?

Yes, they are commonly referred to as break-in or
system burn-in CD’s. They range from highly
specialized (Purest Audio Design System Enhancer,
$110) to a single track on a test CD (Cardas/Ayre
System Enhancement Disk, $20 or XLO/Reference
Recordings Test and Burn-In CD, $27).