Originally Posted by bossobass
For, example, in my tests where we measured voltage out of the SW jack and out of a PEQ after the SW jack, we tried many combinations of MVL and SW trim level with varying amounts of PEQ boost within the subwoofer bandwidth. What we found is that when, say, the MVL is at reference and you bump the SW trim to 'run the sub hot', there comes a point where the voltage no longer increases as you bump the SW trim. That basically meant that the systems SW out signal reached a ceiling and was clipping. The further you bump the SW trim beyond that point, the worse the clipping would become, as common sense would suggest.
There are no clip indicators on consumer gear. So, the average user who bumps the sub "11dB hot" is almost certainly sending a clipped signal to the amplifier, but has no idea that's the case.
All amplifiers have a sensitivity rating. That number tells you how many Vi (volts input) are required to drive the amplifier to full output power. A general rule number is 2V. That is, if you send the amplifier a 2V peak signal from your SW output of the AVR (plus any outboard gear like DSP EQ), the gain attenuator control knob (what most people call the 'volume' knob on the sub amp) can be full open (when turned fully clockwise). This is technically referred to as being set to minus infinity attenuation, or no attenuation. As you turn the amps gain control counterclockwise you attenuate or reduce the gain, but, the whole range of fully counterclockwise to fully clockwise is within safe operation for the input voltage.
Your calibration of the subwoofer to flat with the satellites should be with the amps gain control around 12-2 O'clock and the SW trim around '0'. You can then 'run the subs hot' by turning the amps gain knob clockwise all the way up to fully clockwise with no ill effects because the input signal is not changing.
Conversely, if the amplifiers input sensitivity and input voltage are matched and you choose to run the subs hot by bumping the SW trim/MVL/PEQ Boost (increasing the V of the signal to the sub amp), you risk several consequences, none of them desirable. First, as our measurements showed, the signal from the AVR/DSP, etc., to the sub amp was easily severely clipped when playing scenes from movies like WOTW in extreme settings conditions (such as +10dB SW trim with +3dBRL MVL, EQ boost, etc.). Second, since the amplifier only requires 2V input to reach maximum output, sending a signal with 10V peaks will cause amplifier protection circuits to kick in (clip limiters, current limiters, voltage limiters, etc) because, regardless of where the amps gain control knob is set, you are severely overdriving the amp. Worse, the amplifier is now amplifying a clipped signal, again, regardless of where the amps gain control knob is set.
With commercial subs, there are no indicator lights to let you know when protection circuits have kicked in, preventing damage due to compressing/clipping/over temp/over voltage, etc. Even if there were, they would be on the back side of the sub where you generally won't see them anyway. Of course, it stands to reason that cheaper sub amplifiers have less effective protection circuitry and when the sub is faced with this sort of abuse, the amp fails (MFW-15, Ed, etc.). But, in the case of a good amp with good protection circuitry, just because the protection devices are preventing disaster doesn't mean the resulting playback is a good result. Distortion, compression and clipping are all bad things to be avoided and have been discussed here from the beginning of the forum.
If you have a commercial sub and a popular AVR with no PEQ in the chain and you calibrate flat with the sub amps gain around 12-2 O'clock and the SW trim at '0' or less, then bumping the sub a few dB using the SW trim will not be a problem. If you begin with a system that isn't properly gain matched and you run the SW trim up +11dB and you have any sort of PEQ boost in line, you will very likely experience negative results.