Just bought speakers rated at 100 watts RMS. I have an old 2 channel amp that does 75w RMS per channel. Is it ok to use an amp that outputs lower wattage than the speaker is rated as?

 

Yes it is fine, distortion is generally the cause of speaker failure and that can happen in any setup. The tuning of an amp is the most important part. If you try and make a 75 x 2 amp behave like a 100 x 2 amp then will have the same problem as if you had a 100 x 2 amp that you tried to squeeze a little more out of and make it act like a 125 x2 amp (distortion).

If it is a good, quality amplifier it is more than likely underrated and will deliver closer to 100 than 75 anyway. GL