dmm method for setting gain

I have a quick question about gain setting with a digital multimeter.  I've read alot of posts regarding this topic on this forum, so I think I have a good understanding of how to do it, but my question concerns the results I'm getting. 

I'm measuring voltage from the speaker outputs on the amp using test tones.  I'm using a 70rms 2-channel amp to drive front components (4 ohms).  So I get a voltage target of 16.7V using the formula V=sq.rt.(P*R).  When I measure at different test tones I get quite different readings.  With a 440Hz test tone, I might get a reading of 12V, but with a 100Hz tone I get a reading of around 17V  Is this what I should expect?  And if so should I just use the lowest frequencies (highest voltage) for setting the gain?  Thanks for clarification.

Yes, what you said you are doing is correct.  You find the frequency, within the drivers range, that is the highest voltage level and use that as the limit.  But your example is a rather large difference between those two frequencies...and since 100 Hz is going to be on the downslope of your drivers' range, I think I'd throw that reading out.

I would suggest you also use Plan B and listen for the clipped signal.  Around 400 is easier to listen to, 1k is the standard.

Thanks for the replies.  As always, this forum is a great source for good info.   Using 440 and 1000, I do get much more consistent readings.