So simple question. I want to use hypabass on the kick drum to give more bottom end, after watching YouTube videos and reading up on it I know the infrabass can damage subs that don’t go lower than 35 Hertz. My question is can I use it if I just ignore the infra base knob and just use the sublevel knob in hypabass?
According to the specs, that sub is rated down to 30Hz. RCF did a terrible job as far as the specifications and don’t actually specify the -3dB and -10dB down points nor do they provide a graph to give an indication of the actual low cut off point where the subs unload. I might make an assumption that they actually have a cut off below 30 Hz. My suggestion would be to get in touch with RCF support. That said, with the very generic 30Hz rating, it certainly couldn’t hurt to experiment a bit. Since the speakers are fully processed and protected, they should know when they need to limit if they are reproducing too much low frequency content. They should have an adequate built-in high pass filter, along with that limiting, even a possible variable high pass that changes based on frequency content. Again, another question for RCF support. If it were me, I wouldn’t hesitate to play around with it a bit. RCF as a well-known and reputable company, and I would imagine those boxes are designed for some abuse.
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Thanks for your very clear cut reply. To confirm what your saying about these subs specifically, is that even if someone jacked up the infrabass knob, theoretically the subs shouldn’t allow that lower signal through and not be damaged?
That would be my assumption with fully processed, self powered quality subwoofers by a respected brand. That said, I would exercise caution when experimenting until you know for sure.
just ease it in, and use your ears. the subs will tell you if they’re not happy 
Adding to the above answers, the 30Hz spec on that HDL38 is likely the -10dB point. See the response graph below (if it uploads from my phone correctly). RCF has highpass filters in their subs, so no need to worry about feeding it something it can’t handle. If you’re using the built-in DSP on the box and not RDNet, make sure to select a preset starting with S, the L presets might be high passed at 35 or 40 Hz, I couldn’t find that spec, but based on other RCF subs that’s the trend.
Also a note on Hypabass - from my experience on 5 string electric bass, the Infrabass knob needs a fundamental frequency to lock onto before it will add a lower octave. I haven’t tried it on a kick drum, but I imagine it won’t track well if at all. So yes, leaving the I Infra knob down all the way and turning up the Sub knob is probably the best route. Try it and tweak til it sounds good. Have fun!