Hi Ondra, I've got the same problem with that map as Marcus does in that
http://forum.corona-renderer.com/index.php/topic,2716.0.html post, to clarify things - I'm using Max Design 2014 and corona's dailies (Feb 26 2014 one currently), every rendernode is sse 4.1 capable so no legacy ones. Exactly the same thing happens in my case :( though I cannot say those materials become very dark at the start - they just are getting darker and darker with every new pass from dr nodes until they become darker by some considerable degree . Maybe on some higher pass count that darkening would become even more apparent or maybe if more nodes are used (currently I'm using only 3) the more darker it will get, which would be quite logical as they contribute more and more darkened passes each . Worst of all is, that because of those darkened multitexture using materials, all passes coming from nodes are darker all parts of rendered image become darker as the render goes on, so currently that map is unusable in dr rendering yet renders fine on single machine. I've got some bad experience with multitexture map while using v-ray some years ago - if I remember correctly it was related to normalmaps not working correctly when used in conjunction with it - but that was quite a while ago and I thought such stupid things are a thing of the past and though that may be so, somehow I'm losing confidence in it again :( so I'm going back to using Multi/Sub-Object material - it is of course, way more cumbersome workflow yet way more predictable/controllable and stable...
Some thoughts :
Maybe it would be better to add support for built-in multi/sub-map or VRayMultiSubTex or even make some corona's one with multiple texture loading capabilities as in multitexture but with individual map control after loading them as in multi/sub-mapor VRayMultiSubTex. Plus it would be great if that new corona's map could do some loaded texture's colour, hue and brightness variations as Forest Pack's Forest Color, GroundWiz's Variation Map or Multiscatter's analog does which would be tremendously good for some exterior vegetation and in some cases would make corona's CScatter even more usefull alternative to all of those. It would be even better if that map could show all those used/loaded maps in viewport at the same time as no multi/sub-map nor multitexture nor VRayMultiSubTex can do that and is possible only (as tedious as it may be) by using Multi/Sub-Object material.
The more I think about some separate corona's multi/sub/variation map the more apparent it becomes that such map is needed, cause till corona got none then you'll constantly need to add support for maps mentioned above not to think of some new and exotic ones of which I'm not aware of, and to think of their constant new features/bugs/updates and such + corona's new ones plus users constantly nagging you for adding support for them or fixing some new bug's/incompatibilities or even worse if some of those exotic maps developer will lose interest in their further development and all your work making them compatible would go for naught - I would go crazy for sure :) So I think corona's own multi/sub/variation map would be way more controlable solution for both you and the users not to say it would add greatly to corona's quite considerable usefullness.
Maybe I should post that as a feature request from the start, but somehow I've got carried away and wrote this much here :)