Do you have another synth you can try other than SampleTank? BIAB included CoyoteWT. Try loading that and see if you get sound. If so, we can go into the SampleTank stuff further.

As far as latency, you will have a problem using any softsynth. Many people use a different Midi driver - ASIO. It seems to do much better with latency issues. Do a web search for ASIO4ALL. Lots of posts here about using ASIO.