First, make sure that you're using ASIO, otherwise you easily get a few hundred (!) ms extra latency.
For both audio quality and (given the numbers that you just posted) also latency I would really suggest that you do everything inside Stereo Tool. So the Stereo Tool output would go directly into the transmitter.
In general, read this post for info about needed hardware (both pc and sound cards are covered):
viewtopic.php?f=22&t=4922
If you do this and you use the composite clipper in Stereo Tool, you can easily get multiply dB's louder (and much brighter) output than in your current setup.
Latency can be set as low as 17 ms, but it won't sound too good. For better quality, you need at least 28 ms, which might be too much for your dj's. Stereo Tool can generate a lower latency (~ 11 ms) monitoring output, but for that to work you need to send it to the same sound card as the FM output (it must be one card for ASIO). In many cards you can use the analog output for FM and the digital output for monitoring. Alternatively, for many cards (I know that this is true for the Marian cards), if you plug in 2 cards and connect the clocks, the ASIO driver will see it as one card, so that's also an option.