I have two RELs. Your problem is likely that this is not a punch and slam bass speaker. RELs are music audio system devices. That's why your getting the detail and textures of, say, bass strings or low organ notes or the skin of a bass drum. This particular speaker was considered entry-level about 15 years ago. It's -6db at 25hz. You should be able to find a way to turn it up to a level that it creates boominess and sounds like it's stepping all over you speakers. To to that, turn the crossover all the way up to 120hz and you hi/low level control all the way up, Should sound righteously horrible, but you'll hear it. Are you connected with the Neutrik Speakon connector? Check to make sure you haven't accidentally connected the black and yellow to the same speaker output on your amp.
Getting another REL will not give you the punch and slam. It's not how they are designed. I'd think Rythmik would be a cost effective solution for you. I've not heard one but do understand what they are designing their subs to do. I'd expect it to provide all of the punch and slam you could want, tho you will surrender some of that musicality you're appreciating. If this is a music system I'd suggest considering this as an evolution of your ear. If it's a home theater system then it's the wrong speaker. If your system does double duty then you've a priorities decision to make.
Good Luck!