The Tsukuyomi basically has the same function as the Amaterasu in that it makes your instrument much, much louder, which is nice. The main difference is in the extra tone kick, designed to operate in mids territory instead. If you feel your tone is flat, lifeless and lacks presence, the Tsukuyomi just might be the slap in the face your amp needs. J/k, please be gentle with your amp.
The Tsukuyomi features a 26dB, flat-ish, gain section in series with an optional 12dB of mids gain in the 880Hz-1KHz band.