It might not take as many sealed 18"s to hit 7 Hz as you would think. It depends on the room size, and it also depends on the drivers used. The Re XXX drivers or the Stereo Integrity HST18, for example, have a ton of displacement. Depending on the room and the listeners distance from the drivers, you could probably get some infrasonic sensations with only a handful of such drivers. Given the scenario the OP outlined, that of laying down on a bed of woofers, I don't think displacement will be a problem. You will want very good drivers for such a project, as it would be easy to generate louder harmonic distortion than fundamental in chasing after these frequencies. You will need multiple drivers so as not to push any individual driver so hard that THD becomes a factor.
There is still some serious challenges that have to be met though, even after getting the requisite amount of displacement for necessary SPL. First, you have to have a way to measure subsonic output, so you need the right mic and measuring equipment. You will need to preserve the signal chain for ultra deep bass. Many components such as cheap AVRs can roll off deep bass below 20 Hz. You have to insure no part of the signal chain rolls anything off. However, if all you are doing is playing tones, that shouldn't be a problem, just connect a pro-audio sound card to the amplifiers and play a tone generator.
One thing the OP may be mistaken about is the physiological effects of 7 - 14 Hz bass. I have never read any account of it being healing or relaxing, and I have been over a good chunk of the publicly available literature. On the contrary, I have read accounts of unpleasantness and so on. From
wikipedia's page on infrasound:
"The presence of the tone resulted in a significant number (22%) of respondents reporting anxiety, uneasiness, extreme sorrow, nervous feelings of revulsion or fear, chills down the spine, and feelings of pressure on the chest." Some supposedly 'haunted locations' were found to have the presence of infrasound frequencies which were thought to be the true source of unease and paranoia.
In the end, it would be an interesting project and not hugely expensive. I would start out with maybe three or four HST18s and feed them about 1kW each. That wouldn't set you back more than $3k, not including the appropriate mic you need to insure you are cleanly hitting target frequencies and SPLs.
