Awesome! Can't believe I'm getting an Nu4-6000 for just tree-fiddy.
^Above: Yes, nu6000dsp is perfect for 4 SI 18s. I have D4s so wired series to yield four 8-ohm loads, then paralleled two per speakon so the amp sees two 4-ohm loads. Thats ~1000 real RMS watts per driver give or take, and seems to be about a perfect match.
^Above: I find the built in dsp to be awesome. I have a plain dsp1124p as well, but a nice usb interface with software on computer is cool, as is the ability to do more fancy stuff than just eq, like for example simulate a Linkwitz Transform via shelf filter + PEQ. What exactly do you mean by "subsonic" filter? High pass and/or low pass? Yes for sure, 12 db/octave and 24 db/octave, with the caveat that 20hz is as low as you can go. (You can however push that lower with filter combinations: search the post by LTD to find out how).