Hey guys! I stumbled upon this thread while doing research for my website. This is a really cool site and I look forward to exploring it more! Anyways, I am the creator of The Missing Link! My name is Ashton. I thought I'd post on here in case anyone had any questions.
Also, in response to Natura Sonoris, it is true that (in the example you used) 6hz is ultimately 6hz, but there are a few other factors that are worth taking into consideration when comparing brainwave entrainment products. One example is that holosync uses binaural beats, and many other products use isochronic tones or monaural beats, as these are received differently by the brain (binaural beats are quite different than the other two), this affects the experience of the listener. Binaural beats have their own specific magic, however, that Holosync takes advantage in their strategy of "lowering the carrier frequencies". This is a strategy that, to my knowledge, only Holosync and my own program use.
Another consideration is the soundtrack. There are debates on whether the soundtrack itself actually detracts from the brain's ability to fully entrain to the frequency. But there are also further brainwave entrainment methods that can be embedded in a soundtrack to further enhance the main delivery method (binaural beat, isochronic tone, etc.) A few example of this are volume modulation, where the volume of a certain frequency band in the soundtrack is subtly moving up and down at a specific rate, or panning, where a certain frequency band in the soundtrack is subtly panning from right to left in your headphones.
These differing factors can turn the creation of brainwave entrainment audios into somewhat of an art (or at least a hobby :)), and can lead to varying levels of quality in the market.