Dual mono with REL Subwoofer

[PeppaPig]

somehow I feel the bass is not synced with the main speaker. 

Did you do the step by step REL set up procedure? If not, start again. I've never set up a subwoofer but from what I've read, if you don't follow the procedure to the letter you will never get it to sound right.

Just a hint from Jim Smith, who uses REL subs. Instead of using bass heavy music, he uses a solo female voice as a reference. After all, the voice is where the music is.

Hi Pim, no I didn’t strictly follow their instructions but about 70% followed

Now my problem is the humming sound. I tried many way and I can’t not address it, it’s extremely annoying. Tried same switch board for one of the companion amp and sub doesn’t help... anyone has any ideas?