I have the BassIQ and the Octabass, and they're both very solidly built. No bass loss at all. The IQ, to me, has a smoother sound than the Q tron. But people on this forum have commented that the IQ is too clean sounding. I'm not sure about the HAZ Mutronit's not like the old ones, which did vary quite a bit (only have guitar exp. on these) but it does offer more sounds than the others. If you wanted to go the lots of knobs route, I'd more seriously look into the ModMax Filter by Studio Electronics. Overkill, but many great sounds (may be my Xmas present to myself).Moog has some filters to look at too.
The octabass allows seamless octave blending and tracks really well. I've just gotten a used one and am getting used to when I want to add it in. The octave notes have the same texture as the fretted notesexactly what I was hoping for. The only other octave down that I've used is on my TC G-major. This tracks almost as well, but can add a high pitched hum to the mix, and even at 24 bit losses the textures.