If it's running OK and sounds fine - stop worrying. Unless you're running the amp on full power you shouldn't be anywhere near damaging either amp or speakers.

There are several ways of measuring amplifier power, both using watts. Peak Power is the power of the maximum transient it can put out (based on the voltage at the top of a sine wave), RMS (Root Mean Sqare) power is based on the average power delivered - 50w Peak is equal to about 25w RMS.

Most amplifier manufacturers tend to be rather over-optimistic about the power output of their product and you might find that your amp doesn't actually produce anyway near it's claimed output.

The other thing is that for most music the maximum power is only produced very ocasionally and even running at full 50w power, the average power is unlikely to exceed 20w - unless you're inputting something like a constant sine-wave test signal.

Running 4-6 ohm speakers on a 8 ohm otput shouldn't cause any problems unless you're using high power levels - if the amp appears to be getting hot, there could be a problem, but usually there's plenty of spare capacity in the power supply and output circuitry to handle this at medium power levels. Also using lower impedance speakers would probably produce a higher output volume, which you would probably compensate for, by turning down the volume control.

IF IT SOUNDS FINE WITH NO DISTORTION AND THE AMP ISN'T OVERHEATING, THEN YOU SHOULDN'T HAVE ANYTHING TO WORRY ABOUT.