Disconnect and unplug everything. Now working backward from your amp, one component at a time, connect RCA from pre-amp to amp, turn it on - any hum? If yes, try a cheater plug on the pre-amp to see if the hum goes away.
Assuming not, now connect a source, etc to the preamp. Any hum - same drill as above. Sooner or later this process will lead you to the hum's source and help you determine if floating the ground is a solution.. Don't overlook the antenna as a source of the hum - that is very common. Don't overlook the possibility of a hum occuring because of cable's being too close to transformers in your components (very common source) or other electrical sources. Don't just turn off components, actually unplug them and disconnect all of the connecting cables.
This may not solve the hum problem but you will have at least identified the source and then be able to figure out how to solve it.