You are worrying about shit which does not matter. You don't "know" anything about the performance of a 20.1 if the results you are citing came from a clamp test. Stop paying attention to them. They do not mean what you think they mean. You are misinterpreting the results, and by that I mean you are trying to draw useful information from a flawed test methodology. Toss the results out the window and ignore them. They mean nothing. You are also not going to run 18V in anything but a comp vehicle (well ok, you "could" run higher voltage in a daily system but it's stupid and pointless), so I don't see how that's even relevant aside from the fact clamp tests themselves don't provide any useable information. Also, setting a goal for your system of providing your subs "rated power" is a naive endeavor. It does not matter for daily listening. For starters, rated power is typically a thermal not mechanical limit as the mechanical limit is related to the enclosure used and other factors. Rated power is not a power requirement. Related to that, you can have a rather large deviation from rated power without significantly affecting audible output. So who cares if you apply 25% less power to the sub? You will not hear it. None of the regular quality, run of the mill everyday amplifiers that 99% of users own are going to have a large enough difference between rated power and actual output to cause an audible difference. So as stated, these "clamp tests" even if accurate (which they are not, nor do they provide useful information) would just be a dick swinging contest...absolutely zero reason not to just go by rated power as they will be close enough to actual output in almost all instances that audible performance will not be affected. In the realm of SPL none of the tests are conducted in such a manor to make them a reliable or valid means of comparison, so again not useful.