Speaker or amplifer

That explains a lot. The REL has an 8” driver and only goes down to 30Hz at -6dB so is more like a woofer than a subwoofer, which explains why you’re not getting much slam. All you probably need is something like a Rhythmik F12G to get you what you’re missing.

https://www.rythmikaudio.com/F12G.html

If a Hegel demo is available I really do not understand what the problem is. 
Demo the damn amp. 

I always built a system from the speakers on up. But the only opinion that matters is yours.  

That’s the way I’m leaning towards starting with the speakers.Then I will worry about the amplifier.