I agree with "rharv", you seem to have a routing problem. I have just tested RealBand with Sampltank 3.5 as a VSTi in a midi file. In RealBand, I deliberately set Coyote WS as the default VSTi Midi Synth. The Midi Driver was set to MME. I changed the Ports for a number of tracks to 17 and assigned one instance of SampleTank 3, as the VSTi Synth, to the above ports. Then I assigned ST instruments to match the Channel numbers of the various RB tracks using Port 17.

This worked as designed. It is unnecessary to use ASIO drivers unless you have a need for reduced latency.

I hope this helps