Vasser77
Before you do anything suggested, we need to see if you get a good impedance match with all this.
Your SF3 is tube and may not be a great impedance match with the combined load of your main amp and both subwoofer amps, and you may not get the best of your system.
To see/calculate this for you I need to know.
1: The output impedance of the SF3
2: The input impedance of the main amp.
3: The input impedance of each sub amp.
Cheers George