I used the NastyLF forever, it doesn't work in the same way as a harmonic exciter, it just boosts the low end at the frequency that is chosen and adds some distortion, it doesn't just boost the harmonics. So it still rapes my headroom
It probably doesn't help at all because it's not really like maxxbass, nor free, but if you ever want something cheap for sub harmonics try lowender http://www.refusesoftware.com/lowender.html
can be cool in moderation