You could try a Blackbird or Raptor system from Dave at bossobass.com. He goes by bossobass here on the forum. Jeff (JTR Speakers) or Mark (Seaton Sound) could probably provide you with a passive sub. They use amps from Speaker Power and maybe they could use a rack mount DSP amp from Speaker Power instead of the plate amp. If you only want down to 40 Hz, the JTR Growler is ideal and it is passive. Although it isn't shown on the website, I think you can get them custom with veneers for a more upscale look. Danley Sound Labs has some passive subs, but they are probably larger than what you are looking for.