Thanks for the replies. LumaDB at default settings is very bandy. GradFun3 (mode=6, ampn=4, staticnoise=true) is looking very promising! It previews more nicely than everything else I've tried. When I went to encode in MeGUI so I could upload some further tests to YouTube I was mysteriously struck by
this problem so I'll report back about the colorspace stuff after I've sorted that out.