I am reposting a line of comments I made about this subject on a group on Facebook
The limiters are very safe but you have to take care! I use the clip limiter as a 0 time attack, infinite to one ratio peak limiter, sometimes I let soft clipping disabled. This is a voltage limiter and it should be set a bit higher than the peak power of the speaker, or about 6-8 dB over AES RMS power. You will have to use V=✓(P*I ) to find what voltage you need. Next, the peak limiter is set with a time constant high enough to let the transients pass without problems, but low enough to protect the drivers from over excursion. Take notice that this is working as follows: if a signal is stronger than the Threshold, the limiter is engaged instantaneous and the attack time is the time it passes until the signal is reduced from it's original value to the Threshold value. I am setting this one with a time constant =1/2 of a single oscillation at the frequency where the excursion is the greatest in the bandpass. So if the excursion is max at 50 hz, I put 1/50/2=10 ms. Some are using the frequency at the loads but sometimes this is not good because for example in a reflex enclosure, the peak excursion I'm the octave above port tuning is exactly one octave from High pass frequency and this means it will let it go over once before engaging. The release is a multiple of the attack , 10-20x for lows, 4-8x for mid etc but you'll have to play with it cause it will sound good on some types of music but not on the other... The voltage for this one is based on the way the driver is loaded in the enclosure and the maximum excursion allowed, but if you don't know that, you'll need to put it between 3-6 dB over AES power.. The TruePower setting is for thermal protection. Depending on the type of music you are playing the settings are different. For long duration bass lines ( dubstep, trap etc) , put 1/4 of the AES power with a longer time constant, soft knee disabled (3-6 seconds atk, 12-15 s release for 4.5 " 1700 Watt voice coil) .it will be louder for longer but then it will limit harder. For techno, trance , dnb etc, you need to put a higher threshold 1/3 - 1/2,5 but with a lower attack time (0.5-2 s atk, 2-4 s release). It is important to know that the limiters are safety nets, not meant to be engaged all the time. The are there to save your equipment when a part of a song is a tad too loud, or you need a moment where you increase the level for Dynamics. But if they get engaged all the time, you need more equipment. The gain topology you choose has to be set so the limiters are not interfering with the music too often.
The TruePower limiter is a setting which is using the real power transferred to the driver by measuring the real voltage and current ( disregarding the reactive part) so it has to be set in relation to the number of drivers linked in parallel on the same channel. If you have a 1 KW AES driver you will set it at 250 W, if you put two of them on the same channel, you set it at 500. The peak limiter is a voltage limiter and it is independent of the number of drivers linked in parallel. The same applies to Power vs V@ 8Ohms.
When I am setting the TruePower Threshold, I am usually also measuring the magnet temperature and voicecoil reference impedance and I try to keep the temperature rise under 150°C and the magnet under 80 degree after 2 hours of low crest factor music. Another thing I am doing sometimes , especially when I have the time , space and conditions (so not many times now) is to measure the acoustic power a subwoofer is generating. I am putting pink noise in band with all filters and processing activated and I turn the gain until the TruePower is measuring 10-20 Watts on the output. Then I measure the acoustic power of the device with a method , I compare it to simulations in Hornresp and Akabak and I calculate the Input/Output mean efficiency. If it has a mean efficiency of let's say 10%, I'll add that power to the True power Threshold , because that energy will not be transformed into heat ,but in Sound. Another thing is that when you couple more subs , they increase in efficiency so less power is turned into heat. This means I can increase the threshold once more.
I got my hands on a some Klippel LSI reports from some drivers made by some well known manufacturers and there Delta Tv is the voice coil temperature rise from base which can get as high as +180 °c . So the drivers voice coil temperature can easily get to 200° C but not the magnet's temp. That one I like to keep it under 80° . If the temp is of the coil is kept under 170 ° at all times , the Re of the driver is kept under a certain variation and the power compression stays under 3 dB .
There is something I would love to see more in the processing units: a limiter with a frequency dependant Threshold. A limiter with an output Threshold and given attack and release constants that are not engaged ,even if passed, if the frequency is not in the right band. Powersoft does have this, I think Linea Research has something similar and I've seen this behavior in Eminence D-Fend.
This type of limiting is letting one to better protect the drivers from over excursion in subwoofers. If you know the impedance graph (which correlates to the excursion vs frequency) one can set the limiter to trigger earlier in a certain band. In the previous example, a reflex enclosure has its second impedance peak at 50 hz, where the excursion is the greatest, the FS is at 38 and at 33 is where we need a crossover to keep the excursion under FS at the same maximum level as it is at 50 hz. If there is this type of limiter, we can increase the crossover frequency and we can put a higher sensitivity/lower threshold in the 50 hz band. If the amp allows it and the driver doesn't melt, one could increase the level in the rest of the band a lot. Off course, the frequency response equilibrium would be a problem but that would be another topic. This same type of limiter helps the Powersoft amps to be louder and safer than anything else on speakers with passive crossovers.