All of the synths seem to be excessively low in volume.
I've played with the mixer settings for my soundcard, but the volume on the synths (even when the volume on the bristol synth is set to max) is really to low and I have to turn up my speakers to the max, and even that isn't enough..
It is odd that all the synths should have such a low volume - what version of bristol do you have since the later releases (late 0.10 and most of 0.20) should have lifted the internal signal levels of most emulations.
Either way, it should not be too important. Bristol has always had a fairly low signal output, it is kind of like "works on my system". For other installations then you can adjust the gain of any emulation with the -gain or -outgain option, some users have reported needing up to 64 or more. The one you want depends on the version you have: early releases only supported a single gain stage and it affected all emulations. newer releases support -gain and it should be configurable for each GUI you start.
It is noted that if you had an old release and install a new one then be careful with this gain option, the newer code should have stronger output signals so you may not need the gain or if you use it then you may start clipping immediately. Just lower the soundcard output when you change releases.