1.) What are you running the stratum proxy on? If the machine hosting the proxy is slow and underpowered it can restrict the speed, especially with two cubes running via 2 proxies. Your log output looks normal and working correctly.

2.) Have you tried just running one cube to see if you get full speed with just one?

3.) What is your internet connection like?

4.) The PSU's you are using should be fine, they are capable of supplying 38A on the 12V rail which should be enough for high clock , just, but a better PSU would be advisable for high clock. If you are trying to power two cubes off one of those then it's underpowered.

5.) Make sure no Android devices are connected to your network while mining, this can affect the speed.