The 990 is not THX certified. The Sherwood P-965 design on which it is based is also not THX certified, but apparently when Etronics (Sherwood's parent company and the manufacturing entity that builds both the P-965 and the 990) developed the 965 they wanted to leave themselves the option of pursuing THX certification. Had they gone through that process, then they would have been able (required, actually) to include THX processing modes, hence the THX indicator.