Have not heard the hydra 8 but own the 4. I bought it because a fellow audiophile along with his friend did compare the two and preferred the acoustic 4.
I would recommend it for cd players, preamps but not for amps. The dynamics will suffer if you use it on your amps. At least, that was my experience! The acoustic 4 does what it claims to do but its virtues i believe are best served if and only if you have a sophisticated enough system to reveal the benefits. Its gains are fine and i think for the very discriminating audiophile only.
I would recommend it for cd players, preamps but not for amps. The dynamics will suffer if you use it on your amps. At least, that was my experience! The acoustic 4 does what it claims to do but its virtues i believe are best served if and only if you have a sophisticated enough system to reveal the benefits. Its gains are fine and i think for the very discriminating audiophile only.