I tossed this out there just as a jab....
I honestly couldn't say whether this is fact or fiction on the underpowering, there is so much back and forth on the subject.
I wish I had the test equipment to be able to do the testing myself and prove the outcome one way or the other.
I generally tend to get an amp rated higher than my subs just due to the fact that music does not generally contain a flat non fluctuating signal as the signals used to determine/verify rms on subwoofers. (pink noise)
A signal producing 50% power and 200% peak power evenly equals 100% average which could be recognized as rms.
A 100watt rms sub getting 50 watts with 200 watt peaks evenly = 100rms, now if your amp is rated @ 100 watts and just barely makes that mark, with a supply voltage drop(if) you wont even hit that mark, but have a good chance of having crappy sound.
However if you have an amp rated to do 150 rms with the voltage drop and everything else you will have the ability to provide your sub with the needed power and have some extra for those musical peaks.
(The wattages above are just for illustration for ease)