Grounding ?

---> Bob,
why relying on grounding through mass of interconnect cables is bad idea?

I have preamp and two monoblocks - all with unbalanced connection only. Ground is connected to the chassis internally. I have mains cord grounded only in preamplifier. Both monoblocks have lifted ground in power cords and are grounded through interconnects and preamplifier's power cord.
All hum disappeared and such connection seems to be safe (monoblocks are grounded through preamp). What is wrong in such approach?

Thank you for the answer.
So, what is a solution?
Is the isolation transformers like Jensen Transformers are the only solution for full unbalanced system?
Honestly I do not know any person who use it.