ADX16 encodes additional range and precision as compared to ADX10. The density range encoded by ADX16 is about 8 vs about 2.046 for ADX10. Since no film will have a density of 8, this means scans in ADX16 will appear very dark if you just look at the DPX files directly. The ADX16 IDT takes the range into account when converting ADX16 images to ACES.
It sounds like your 16-bit DPX files are encoding less range than is specified by ADX16. This is likely why you’re getting very bright ACES images out of the IDT.
The IDT is needed. If your scans encode the same range as ADX10 but use all of the additional 6 bits for precision, try using the ADX10 IDT.