Hi,
If you have the volume fully turned up, that is the same as the fixed level.
Have any mods been done? Some can reduce the volume and increase the output impedance.
If it bugs you, one answer (not the cheapest) is to add a buffer with some gain. Wayne at Boulder Cable can modify a Burson Buffer to any gain level that best matches the rest of your system. In the process, the SB3 sound will move another level.