These were fun to play with yesterday too. For rolling off rumble and adding air. I loved passive for air, active for tightening the bottom. Just add pigtails, and psu if necessary. $300 each. They come with pinouts, VFX-2 are passive, VFX-200 are active. Schematics are $45, you gotta order those from Orphan Audio or find online, I never ordered them personally. Will fit 3RU with modification. 2 vintage inductors per filter. I will keep just one of these for myself, probably have 2 dozen.