I don't know what "normal level" is but if it's, say, half way (15 of 35, for example) on a 4 volt out preamp and you set the gain for a 2 volt preamp signal, then every time you turn the volume past half way up... you cause the amp to clip the signal. To better explain that, I'll assume you set gains at volume level 15 and we'll assume that the preamp voltage at that setting is 2 volts. Now, with that setting, you've set your amp to produce its maximum power with a 2 volt signal and any time the preamp voltage exceeds 2 volts (any time the volume goes up past 15), the amplifier is being asked to amplify the waveform more than it actually can. And since amps are dumb in that they don't know what their maximum is, it will happily comply with that request and the result will be a waveform with the peaks cut off. When that happens, even though the sound you hear may not be particularly loud and possibly not even audibly distorted... heat builds up in the speaker's motor. This is because the amp is pushing the cone out (& pulling it in) and then attempting to stop it and hold it for a few milli (or nano) seconds twice for every cycle. So for a 30 hz note, that's 60 times per second that the amp is essentially sending DC voltage through the coil and when that's happening, the motor is not able to dissipate the heat that builds up. And again, all of this can occur even if you aren't driving the sub hard and even if you can't hear audible distortion.
This is why you read or hear people like me saying that where the gain knob is set makes no difference at all with respect to it being set properly. i.e., you can't think of it like a volume control knob and assume that just because it's only set to 25% that it's somehow only turned up 25%. That's simply not true.
As an example, the gain control on the 400 watt amp that runs my door speakers is almost in the "off" position (if it were a volume control). This is because my processor has 4 volt preamp outputs and the amp only accepts input signals as high as 4 volts. So in the sense you're thinking of the gain, mine is virtually turned off. Yet my speakers get the maximum amount of power the amp is capable of producing, when the volume on the head unit is at max. If my preamp signal was only .5 volts max, then my gain would be set to almost "max", (in the sense of a volume knob), but the amp would still behave the exact same way it does now.
So with all that wordy explanation out of the way... you need to know exactly what volume your head unit clips at (which will be max preamp output voltage) and you need to set your gains with the head unit at that volume. If you do that, you'll have no more voice coil cook outs.
Regarding the infrasonic filter (subsonic is not the correct word)... it is to protect the speaker from unloading at frequencies below Fs (its resonant frequency) and it should generally be used on speakers that are 12" and smaller. It should also be used on some 15 & 18 drivers, although they aren't as prone to unloading below Fs as are the smaller ones. And of course, there are smaller speakers that can handle low frequencies but for the level of sub you're using, you need the infrasonic filter set to just below Fs, even in your sealed box.
As for the crossover, 130 hz is a bit high and if you aren't using that setting to make up for a lack of midbass, it should be set to ~80 hz. When you set it above that on a subwoofer, you're asking the subwoofer to pull duty on frequencies it's not designed to play and that can introduce distortion and tax the amplifier to the point of heating up the voice coil(s) in your subs... even without audible distortion or clipping.
Hope that helps and makes some sense.... I'm still on the first cup of coffee this morning. ;-)
