Originally Posted by SherazNJ
But craig, I'm not using boost from DSP. There is no equalization. Are you saying that any type of boost other than one coming from avr sub trim can cause this issue?
Boost... is boost... is boost. It doesn't matter if you add the boost with the AVR's subwoofer trim, or you use Audyssey DEQ, or you set a House Curve with the miniDSP or you raise the gain of the amp on the subwoofer, or you simply turn up the Master Volume Control, if you increase the signal level anywhere in the chain, you eventually reach the limits of the sub driver. The following graph is your sub from the PSA website. It demonstrates the limits of your subs:
You can see that as you increase the signal level, you eventually reach the point where you don't get the same amount of output increase as the amount of signal increase. In the top graph, (yellow), 5 dB of signal has been added, but only about 2 to 3 dB of additional output was realized from about 30 Hz down. This phenomenon is known as "compression." If you were to add another 5 dB of signal, you would get even less additional output. Eventually you reach a point where additional input results in ZERO additional output.
The following graph is not your sub, but I post this graph for demonstration purposes. The sub is the SVS PB13 Ultra in 15 Hz tune. It shows the distortion levels at 3 different signal levels.
In this graph, you can see that the distortion increases dramatically when the maximum output of the sub is approached, especially as the frequency approaches the port tune. Below port tune, the distortion goes "off the charts" because the driver is no longer "loaded" by the port. Your subs are not as capable as the SVS PB13 Ultra, (they have neither the output nor the extension), and they likely reach the point of high distortion well before the Ultra. This is likely the "bottoming" noise you're hearing from your subs with excessive output below port tune.
I said before, if you add boost, no matter where that boost comes from, you reduce headroom and increase distortion. Based on the compression graph of your subs, it looks like you have about 103 dB of output capability at 30 Hz and 97 dB at 20 Hz. Adding the 2nd sub should provide an additional 3 to 6 dB of output at those frequencies. If you go above that, you'll be introducing compression and distortion. With room gain, your subwoofer system should be capable of about 112 dB at 30 Hz, (estimated), and about 106 dB at 20 Hz, (estimated.) When calibrated for "flat" response, you should be at the limits of your subs with Master Volume Control settings of about -9 at 20 Hz and -3 at 30 Hz. If you boost the response at those frequencies by 10 dB, you'll want to limit the MVC settings to about -19 to -13. That is not a compromise I, (personally), would make, but feel free to set up your system however you chose.
PS. You can see the Data-Bass power compression and distortion graphs for your subs here: http://www.data-bass.com/data?page=system&id=88&mset=95
The power compression graph shows exactly what I described... add another 5 dB of input signal and you get ZERO additional output. The distortion graphs show that the distortion starts to rise dramatically at about 25 Hz. At 110 dB and 20 Hz, the THD is about 28%!!!