I run many softsynths with the KX drivers. I have the same problem when trying to run multiple synths in stand alone mode.
Therefore I use a "host" program Like VSTack. With a host program you can load up as many instruments as your CPU/RAM can handle without getting the buffer errors.
There are many vst hosts out there and you should be able to find a demo or free hosts if you search the web for VST Hosts.
http://www.kvr-vst.com/
theres a great site with all manner of info and links to VST instruments, effects and hosts.
Toad