You first need to get the sensitivity (gain setting) correct.
The method I used, since I use up to +10dB of L/T boost, is measure voltage in a worst case scenario. I played the actual movie soundtrack scenes and measured the voltage at the end of the signal chain (the signal you'll input into the amp). That happened to be 2 scenes in WOTW, the bridge collapse and the plane crash scenes. The voltage measured 9.2V. Virtually every other movie was well below that, so I set the gain setting for safe operation with 9.2V input.
That coincided with 23dB on the dip switches, but in bridged mode (which is the only mode I use with these amps), there's a -6dB attenuation, so the correct setting for me is 29dB.
The front panel gain attenuation knobs go from -infinity to 0dB, but for practical reference, about 60dB of range over 31 steps. That's far more adjustment than any pre/pro allows for and my suggestion is that if you prefer to bump the sub level vs the rest of your system, don't be lazy, use the front panel level adjust knob. When you bump the pre/pro SW trim setting you change the worst case scenario output voltage you just painstakingly measured and run the risk of sending a clipped signal to the amp. When you use the amps front panel level adjust knob, you're well within your sweet spot regardless of the knob position. If you end up with the knob set to -infinity and still feel you need more, increase the gain setting dip switch to the next position. If you do that and see the VPL light flash often, re-think your system, which probably needs to be increased across the board.
Next, set the VPL.
As Ivan touches on, this is a more complex decision than most might first think. I disagree with him when he suggests that this is the subject for another thread. It should be discussed here, regardless of what amplifier you own. Very early in this thread, I mistakenly suggested that setting the VPL to its max setting will effectively eliminate the the VPL circuit, especially in bridged mode, where the value is doubled. Although this is technically correct, it's not a good idea at all.
1) The values listed on the back panel VPL dip switches are peak values. For RMS you need to divide by 1.41 (this goes with Ivan's suggestion).
2) The values listed are doubled when in bridged mode.
3) Setting the VPL dip switch to a lower voltage peak limit only lowers the max output voltage, but... it allows for more current into low impedance loads and runs more efficiently, which practically eliminates the need for over temperature protection in our application (HT subwoofers) and certainly may prevent failure when powering low impedance loads (if a less-than-perfect CPL circuit exists in your clone version, which some evidence suggests). I also posted early in this thread a link to test results done by a Brit pro sounder. Since there is no mention of gain/VPL settings with the clone, it was run in 2 channel mode and it was fed a 40 Hz sine wave with a 2 ohm resistive load, it's failure isn't all that surprising and, IMO, might have been prevented.
4) The CPL (Current Peak Limiter) circuit is not user-adjustable. Setting VPL affects the current draw, so if you see the CPL light flash, up the VPL setting one step and see if the CPL light continues to flash.
NOTE: Again, don't be lazy. Sit right in front of the amp so that you can see which lights are actually flashing or lit. It's impossible to tell from across the room, especially while you're watching your monitor and not your hardware. Play the scene that is causing the flashing lights and deal with it. Why hit limits and increase risk if you don't have to? You may conclude that the problem is rare and properly being dealt with by the amp, which is cool, but run through the exercise to get the system properly tweaked for your app and enjoy the show.
IMO, prevention is the best medicine. These amps are designed for pro sound systems running off 220V/16A power. Running a system with +10dB of boost below 10 Hz, fed by a signal chain that's -3dB @ 3 Hz, playing extremely dynamic source with content to 1 Hz with the amp running off 120V 30A power (double the current) is so far removed from pro sound applications that it warrants some thought and discussion regarding the systems operation, the amps protection circuitry and the proper settings.
Systems and venues that are much closer to a pro sound app, like ported subs, use of HPFs, playing 40 Hz recorded music at a high level for extended periods, etc., place the amp in a situation that's closer to what it was designed for. Switching to the scenario in the previous paragraph is like moving the amp to an alien planet. If you don't take the necessary steps to set the system up properly, you most definitely run the risk of catastrophic failure, and good luck with that.
When I get the time, I'll post a chart with correct gain settings for various voltage input values and corresponding VPL settings, which this thread should discuss more than casually.
BTW, Ivan, thanks for chiming in! Your input is very much appreciated and needed in this thread.