I would buy a 4 ch amp.
You can mix mode your 2 ch amp but I have no experience with your amp running mix mode and I know some can not handle the load of mix mono.
I think your amp will have heat problems in mix mode but if it doesn't it should be better.
Your boost is at a freqency (40 - 70 Hz, some where) which your blocker should block if it cut off is above the boost frequency.
I would get a GM-X524 to go with your GM-X542 and you have a great set up.
