Lonely Planet says that Prasat Chhin (or Prasat Chen) is the place where the statue of the giant wrestling monkeys was found. The statue is now in the National Museum. It may have also been the location of the looted fighting warriors statues. I may have found where the warrior statues were originally located. It was in the back part of the temple.
There is an amazing stepped pyramid at Koh Ker. The seven levels rise to 40 meters. It looks more Mayan than Khmer. Maybe the world was more connected than we presently believe. Koh Ker was the capital of the Angkorian empire only under Jayavarman IV, who reigned from A.D. 928 to 942. His son, Harshavarman I, moved the capital back to Angkor in A.D. 944. It is impressive that so much was built in 16 years, especially this pyramid. The stairs are no longer safe to climb.
It is not possible to do Beng Mealea, Banteay Srei and Koh Ker in one day. However, Koh Ker is definitely not the one I would eliminate. In fact, it is the best of the three. It takes a couple of hours to get to Beng Mealea from Siem Reap. We did Beng Mealea, including the back side in ~2 hours. It took ~1.5 hours to drive to Koh Ker from Beng Mealea. We spent ~2.5 hours there, stopping at several sites on the loop road, walking back to the giant pyramid, and having a quick lunch. I used the time stamps on my photos to estimate the times. Therefore, you are looking at a ~11.5 hour round trip from Siem Reap. I had hired a car and driver. A set tour may be shorter and may take less time. Beng Mealea is 5 USD plus 2.50 USD each way for the private toll road. Koh Ker has a 10 USD entry fee, which you can pay onsite or at the Beng Mealea toll booth.
The main temple area of Koh Ker is quite large. The Canby Publications map is quite good. You approach from the south and there is a loop road. We stopped at the following temples:
1. Prasat Nean Khmau;
2. Prasat Chhin;
3. Andong Preng;
4. Prasat Thom;
5. Prasat Krahom;
6. Prasat Balang;
7. Prasat Thneng;
8. Prasat Banteay Pichear;
9. Prasat Chrap; and
10. Prasat Pram.
There is a 10 USD admission fee. The ticket may be bought at the Beng Mealea ticket booth or at Prasat Thom. Andy Brouwer has a very nice writeup on Koh Ker.
Under construction - It was harder to tell which sites were which on the east side of the Koh Ker loop road.
There are clean public restrooms just up the road north of the Prasat Thom parking lot. They are within walking distance (take the turn to the left).
Under construction - There is a small market between the parking area and Prasat Thom. You can pay your admission ($10 for foreigners), eat and buy souvenirs there.