can I safely drive 250watt speakers with a 600watt

Yes.........

I'm driving 90 DB efficient speakers with 3000 watts.

120 watts would make them play, but they sure wouldn't sound as good with the smaller amps.

It's been said many times, but it's true. You're much more likely to damage a driver by clipping the amps into distortion than from putting too much power into them.

As others said it fine and most prefer much more power than needed.

The amount of power (watts) going into your speaker depends on characteristics of your speaker, and how loud you play the music. A 600 watt amp will deliver only what the speaker demands, not what the amp is capable of delivering. So, the 120 watt amp and the 600 watt amp are probably both delivering something like 10 watts rms most of the time. However, the more powerful amp can swing a higher voltage, which if it persisted, would corespond to more watts. But, for music, such high voltages are brief peaks of the music signal waveform, and don't represent much power. The high powered amp will deal with these peaks better than the low powered amp.

So long as the signal is music a high powered amp will not damage speakers. However, if there is a malfunction (IC ground problem perhaps) the high powered amp can toast your speakers quicker than a low powered amp.
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Go Albert!

Thank you all I assumed so but wanted to be on the safe side

I second what Eldartford wrote. Under normal use it shouldn't be a problem, but under some unique circumstances it could damage your speaker.

A few examples:

- an interconnect is plugged/unplugged with the amp on
- you drop your tonearm and the stylus skates across the record
- your preamp doesn't buffer the tape outputs and you create a feedback loop
- you momentarily lose power and as your system re-powers one of your components generates a turn on pulse

You get the idea. It's not likely, but it's not out of the question.