When I run my subs at reference levels (115dB for 15Hz-10Hz ULF frequencies) the amp clips if the sub level in my AVR is on the + side.
Doubling the line voltage allows me to keep my subs gain at the upper end where they shine, and keeping the AVR sub levels in the - zone with no clipping and more headroom.
If you prefer less than reference audio levels for your LFE/ULF,
you would likely never have a headroom issue or notice the difference it makes being in the negative zone.
For the Best Sub performance out of your system,
I strongly suggest anyone interested read this thread
in particular, this section related to Gain Settings and Maximum Sub Output
II-E: Gain Settings And Maximum Sub Output
There is another aspect to the gain/trim issue that is worth mentioning. Depending on how the DSP in a given subwoofer is implemented, the subwoofer may only be able to achieve max output levels with the gain control set very high. Some subwoofers are only able to achieve maximum output levels when the gain control is set to, or very near, the highest setting. So setting a lower gain control, and a correspondingly higher AVR trim control, might not result in the same amount of peak bass SPL, irrespective of issues of clipping.
Apparently, this issue may be more common in subwoofers with digital (rather than analogue) controls. But, according to several examples I have observed from various threads, the issue is not at all limited to subwoofers with digital controls. Some subwoofers with analogue controls may have the same issue of not being able to achieve higher max output levels with low gain settings.
How important this max output issue actually is probably depends on the situation. For instance, I believe that a relatively lower gain setting might cause a ported subwoofer to chuff prematurely. Again, depending on the situation, even someone who is listening at a fairly moderate listening level, let's say -15 or -20 MV, might experience issues if he were using a significant subwoofer boost, either independently or on top of DEQ.
Putting a sudden peak demand on the subwoofer, with the right low-frequency content, might not enable the subwoofer to access the full output that it is designed to produce, if the gain level isn't fairly high. In that case, the subwoofer just wouldn't play the low-frequency content at the volume it was supposed to. In other words, it would simply stop getting any louder during that peak content. Whether we would even notice that, or whether we would hear the subwoofer make any audible sounds of distress, are separate questions.
But, unless we are sure that our subwoofers can achieve max volume levels with low gain settings, it is probably a good precaution to keep gain levels fairly high. Typically, that will mean using corresponding lower AVR trim levels for our subwoofers. This is not an issue that I have often heard addressed by subwoofer makers, but I suspect that many would intuitively know that some subs produce max volumes only with high gain levels.
This is just speculation on my part, but I think one reason that this issue isn't discussed more is because sub makers are not typically testing their subs as part of a calibrated HT system. So, they aren't dealing with gain/trim relationships at all in their design and testing process. When they want to push one of their subwoofers to its limits, or they want to measure its maximum output, they just max out the gain control on the subwoofer itself. It is only when subwoofers are calibrated as part of an HT or audio system, with an inverse relationship between gain and trim, that this becomes an issue. But, I believe that it can be an issue, and I believe that is another potential reason for attempting to keep gain settings fairly high.
CEA 2010 testing, performed by Data-Bass, always measures max output with gain controls at the maximum setting. And, as noted, some subwoofers may not be able to produce those same max SPL numbers, that we see on Data-Bass or from other professional sources, with lower gain settings. This won't be true for all subwoofers, but as a matter of best practice, I believe that it may be generally advisable to keep gain settings fairly high, and AVR trim settings fairly low, in order to maximize available headroom. An exception to this general policy could be a situation where a lower trim setting didn't successfully power a subwoofer on, when it was set to Auto On mode. But, that would be extremely unusual with Denon/Marantz, or Onkyo, receivers and processors.
[People with Yamaha AVR's are apparently much more likely to experience issues with subwoofers not turning on automatically unless AVR sub trim levels are relatively high--perhaps even fairly close to 0.0. That is due to the lower voltage signal sent from some Yamaha AVR's to the subwoofer. Newer high-end Yamaha AVR's are reported to have addressed the problem.
If subwoofers will not turn on automatically in Auto mode, without higher AVR trim levels, then the higher trim levels may be slightly less likely to lead to clipping issues, since the voltage from the AVR was lower to start with. Some Yamaha owners use a Y-connector into both subwoofer inputs in order to double the voltage going to the sub. And, that typically resolves the Auto On issue. Of course, Yamaha owners can also choose to just leave their subs on all the time, if the Auto On issue proves to be a real problem. That will consume slightly more energy, but will not affect the operation or longevity of the subwoofer.
AVS member @Basshead recently mentioned a clever solution for achieving more headroom, with higher gains and lower trim levels, which seems to circumvent the Auto On problem with Yamaha AVR's. He went from a -1.5 subwoofer trim level to a -4.5 trim level, with a comparable gain boost, and obtained 3db more headroom, prior to clipping. But, his subwoofer didn't power-on reliably when watching TV at very low master volume levels. So, he lowered the trim levels on all of his other channels by -3db, and raised his MV level by +3db, and is now able to have his sub power-on reliably for low-volume TV content, while still having more headroom available for louder movie viewing. This is an additional technique that Yamaha owners might wish to try.]
For those that are happy with what you have, leave it alone,
ignorance is bliss for those that are happy with the results YPAO gave you.
I'm not telling anyone what to do,
I'm giving options to those who are NOT happy and asking for solutions.