The earlier VTF-3s with the 12"s could be overdriven if you pushed them hard enough, at least in 'max extension' mode. In max output mode, I don't think they could be overdriven. I don't think its worth it to spend a chunk more money here, especially when you got a pretty good deal for that one.
I would try to find the sub's limits at this point, so at least you know how hard it can run before getting into trouble. I would start by running some test tones with something like
this. Push the gain on the tone generator to max. Then, with the volume on the sub or AVR at a moderate volume, set the sub to a deep frequency, like say 20 Hz, gradually raise the volume until you hear some mechanical stress, or something like a tapping noise. Try it with other deep tones, like 16 Hz and 25 Hz. There is no point doing it below 16 Hz though, since that is below the sub's tuning frequency.
Try that testing in both max extension mode and max output mode. Remember when you are running the VTF-4 mk4 in the 'max extension' setting on the amp plate, at least one port MUST be plugged, or else it won't be filtered from over-excursion properly. If won't hurt the sub to have any of the ports plugged when the sub is set to 'max output' mode. Don't run test tones at loud levels for long durations, that is very hard on the amp and driver, not just for the VTF-3 but any subwoofer.
Assuming that you had a port plugged with the sub was over-driven, there are a couple things you can do to prevent this from happening again, if you still want to play the sub loudly. Like I said, you could set it to 'max output' mode, which will raise the high-pass filter and limit its excursion more. Set the 'Q' control to 0.3, that will also reduce deep frequency excursion.
The newer VTF-3 that uses the 15" looks to be pretty bulletproof, if the 15h and ULS-15 mk2 are anything to go by. I was not able to overdrive the VTF15h mk2 or ULS-15 mk2 no matter how how hard I pushed them- and that is groundplane testing with an extremely high input signal and amplifier gain maxed out, which much harder than any real world contect would ever push these things. Pretty much impossible to overdrive. The older VTF-3s and older Outlaw LFMs can be overdriven when set to max extension mode, but they do have tp be pushed very hard. The other subs you listed should all be bullet proof as well, SVS and Monolith especially. BUT with any variable tuning sub, you need to have the filter set to match the port tuning, ie., you have to set the amp to match the plugged ports. This is true for Hsu, SVS, Rythmik, whatever. The only variable tuned subs where it doesn't matter are the Monolith THX subs and the Outlaw X13-Ultra. You could have them set to whatever and run them as hard as you want and they will not be susceptible to harm. That's nice and all, but there are also disadvantages to that approach as well.
