Using Avidemux 2.6.12 Windows x64. I'm trying to reproduce a couple of the x265 presets. Some of the preset options aren't exposed in the GUI. Near as I can tell, these cannot be set in ADM.
--amp, --no-amp
--b-intra, --no-b-intra
--ctu, -s <64|32|16>
--early-skip, --no-early-skip
--fast-intra, --no-fast-intra
--limit-modes, --no-limit-modes
--lookahead-slices <0..16>
--max-merge <1..5>
--min-cu-size <64|32|16|8>
--rdoq-level <0|1|2>, --no-rdoq-level
--rect, --no-rect
REF: http://x265.readthedocs.org/en/default/presets.html (http://x265.readthedocs.org/en/default/presets.html)
Why am I trying to reproduce the x265 presets manually? Because the Basic Configuration gives the wrong result. See related thread: http://avidemux.org/smif/index.php/topic,16558.0.html (http://avidemux.org/smif/index.php/topic,16558.0.html)
A lot of the work to bring x265 into ADM was done by Koolaidman.
Not sure if he is working on this feature in the background (I suspect not recently).
I don't think x265 is a priority by the Author/Developer of ADM so you may have to wait a while.