James Harris wrote:


My hi-fi sounds great sometimes and not others.

Most likely causes would be your state of mind and your selection of music
to listen to.

I am told that it
needs an hour's warm-up and this could fit with the times it has
sounded good.

If your system has some technical defect, then warming up could deal with
the problem. In general audio systems are up to peak performance within a
minute or less of commencment of use.

For example, great one evening but poor the next
morning.

Your system could have a defect that makes it extraordinarly sensitive to
operating temperature.

I leave the electronics on but not playing.

Is there a CD that can be played to loosen the system up?

Nothing special should be required.

I suspect the speakers most. Bass extension is particularly absent at
times.

In fact, the cause could be anything else in the system.

Any suggestions as to whether playing a very low frequency sine wave
or similar could do the trick?

Not if your system is in good shape.