The DTS button only offers options for DTS processing modes - which with a DTS 5.1 source means straight DTS and DTS+NEO6. Applying Pro Logic IIx to a DTS source is considered Dolby processing, and as such is only available by pressing the "Dolby Mode" button on the remote. (Interesting side note: unlike with older processors, the 990 will not let you apply DTS ES to DTS sources. This is because DTS issued instructions to manufacturers to only use NEO6 to generate surround back data from DTS 5.1 sources, since the processing used by DTS+NEO6 is exactly the same as the processing used by DTS ES Matrix with a DTS 5.1 source.)