What are you expecting for a blow your mind sound? Just really loud? you will need much more amplification to do that. A -4.0 setting on the Onkyo, I suspect, is pushing the limits of the receiver and the distortion you are hearing is being generated by the amp.
A better sub will certainly help as it will give you more headroom for the explosions, etc. and take some the burden away from the M60s and the receiver. A good will really help fill the room with good LFE and this is where a lot of extra SPL is made.
Just curious what are you using for your surround speakers right now? Are these from a HTiB? If so then yes the QS8's will be a large improvement over them.
