I think there might be a misunderstanding. There is a Preah Kahn in Siem Reap but also one in the jungle. As I understand the tours (minivans) from SR take the route 6 east wards and then go north near Stung to the temple. But according some descriptions it is possible to follow route 66 east wards from SR to Preah Khan.
Last time when I was at the Beng Melea temple they told us their was a route but since it was rainy season it was not usable.
Sorry, my mistake. I wondered already as going westward from Boeng Malea should be no problem. Eastward probably is something completely else and I have heard tales of single tracks and crossing deep rivers. Unfortunately I have no recent news about this road but do a search at Khmer 440 or at cambodia.mellentin.de and there may be something. There is a travel report from 2004 about this trip eastward - see http://www.talesofasia.com/cambodia-ca2-route66.htm by Jim CA2 which may shed some light on it.
On the road in front of Beng Mealea it is a straight shot east into the forest. Probably less than 1km from beng mealea before it turns into an ox cart track through the forest. This was in late 2004 when I did it - haven't heard of any road construction going on up there so it's probably the same.
Thanks for the confirmation , I headed down a trail in that location last October but it didnt look used much so I headed back to the temple and read my Adventure Cambodia book, it showed the 66 being above Beng Melea so I drove around looking for it and of course never found it.
I am heading off to Cambodia again in Feb to try it so will post the results after.