Chris I think the S80 uses a specialised battery/charger. Selfbuilt had described it in his S80 review. http://www.candlepowerforums.com/vb/showthread.php?335133-Olight-S80-%28XM-L-26650-with-integrated-charger%29-Review-RUNTIMES-BEAMSHOTS-VIDEO Check what he wrote right below the battery...
I have the Eagletac 17650 and use them with the Wolf-Eyes Champion, a 2 cr123 light which doesnt accept 18650 and they fit and run fine. HKJs review: http://lygte-info.dk/review/batteries2012/EagleTac%2017650%201600mAh%20%28Black%29%20UK.html
Bought it from Europe recently for 43€ and absolutely like it. I have the low mode problem with alkalines too though (havent tried NiMhs yet). I'm thinking about a diffuser for it, it would make the light more versatile
+1 on what Flashkat wrote about gels, a way to do this: http://www.diyphotography.net/gel-caps-diy-light-painting-gels-for-your-flashlight .
Also, Dereelight have a DBS (48mm) green filter but it may not fit the MT40, perhaps ask them about it?
I have the Jetbeam PA40, which according to the specs has the same head diameter as the LD41 and I use the Olight M21 diffuser. It fits fine. I guess it will fit the LD41 too
I have the U2 version and love it. Well built light, it is not a thrower like the Tiablo or Dereelight Aspherics but great as an all rounder. I just dont like that it starts on high. I plan to get their pro tailcap to fix this
The Balder BD-1P is a good one since it can be AA or 2AA (or 1x14500). Forward clicky without flashing modes. Its mainly floody but great for camping or indoors use.