I looked at the Ultracurve as well, its a high end EQ for sure.
Decided against it because I felt the previous equaliser I bought although it allowed me to mess with the audio, that wasnt what I really needed as it didnt give enough control of the lowest bass frequencies. Besides when running from the LFE ive got my Cutt Off at 120Hz. Thats why I opted for the "Behringer Bass Pro" as it specialises with low end sub harmonics which it achieves very well.
How are you intending to use the "Ultracurve" in your system, which surely is designed more with subs/speakers in mind outputting various frequencies and not really LFE levels for your transducers and not speakers as such? Also curious as to how you will you use it along with the "Pro Bass"? You got me a bit confused here what your ideas are....
My own reason for additional subs was because testing I have been doing felt better with the dual 12" units at each side of the seat than actually at the back. Their is a nice resonance feeling from them which flows into the seat. It isnt as hard hitting as a Buttkicker which work better with low end frequencies, yet together a great combination. I opted for a single larger sub at the back that will be directly behind the seat because this should really now then also give great resonance sensation up the back of the chair into the players spine and back regions.
With previous testing Im trying to avoid just one area having an overly strong bass sensation. I dislike this and found it an issue issue with using a single Buttkicker device on high volume. Most people with experience of transducers or indeed subwoofers would agree that set too high they can become annoying detracting rather than adding to the experience. In theory having 3 subwoofers running at 1/3rd volume will give a smoother and wider spread of bass without the annoyance of one unit pushed to its limits. Then adding in the Buttkicker to provide the very low end punch.
Of course my plan is that the subs and transducers can be controlled with different amplification and settings from the 2x Pro Bass units. This way they can be both optomised to work with different bass freqencues. So far I find the Buttkicker is best with just the real low end bass and rely on the subwoofers for a higher range of bass, besides feeling good in actual audio bass terms its really impressive without even needing to be at a loud volume. I see having this control in both audible bass units with tactile units as major benifit to what my own cockpit will achieve in immersion.
Your amplification for the Buttkickers etc is 4ohms.
I put it too you to consider testing a 12" car subwoofer and run it from your current amps, return it for a refund if your not impressed
Heres 1x 12" at work with some crappy C64 Midi music streaming from Youtube.
The effect is noticable with the bottle and the volume isnt very high....
