it's why i tryied to use the insert output as an unbalanced output, in order to get a normal gain
I'm not sure what you mean by "normal" gain...
Even if you use the insert output, you'd still have the same issues... i.e. 6dB less gain (half) and susceptibility to noise at high gains (because you don't have the CMRR capability of balanced signals anymore).
The balanced driver on the preamp board will A) give you 6dB more ouput, and B) helps you connect to the next gear in a balanced configuration.
You give up all the benefits of balanced connection if you go unbalanced. It doesn't matter where you tap the signal. If it's unbalanced, it's unbalanced. The INSERT OUTs are unbalanced.