Guys , you can probably remove or rename PL's lowres.vol from your data directory and have *compatability* with ROTJ , and still have PL's terrains ..
just a guess
*unfortunately* lowres.vol is an undocumented .vol file that take precidence over rb.vol , rbmulti.vol and I think even rbshell.vol .. meaning the game looks to lowres.vol first , and if it finds the file name there , it won't look in the other ,vol files ..
The *problem* we have is that rb.vol is virtually impossible to recompress in it's entirety , or it comes out 4 times it's expected size .. this may be because of the rather unusual pbmp compression that was used for the textures in rb.vol (rbshell.vol and rbmulti.vol).
Hoping to work this out with others soon, however compressing files into .vol files is definitely the *wave* of the future for SP and MP RBII/3D mods ...