This is less a question specific to the Willsenton and likely more based on general subwoofer benefits based on your room size and integration.
A properly integrated sub in that system will, yes, add more bottom end, but more than anything, it helps to neutralize and room modes, which results in a wider and deeper soundstage and more clarity in the imaging across the entire frequency spectrum. This is the primary reason I absolutely love subs that are well integrated into a system and room.
What this will not do is ease the load your speakers are looking for from your amplifier unless you implement a crossover between the amplifier and speakers. For instance, if you want tighter, faster, more articulate bass and musical midrange, you may be better off going with an amplifier that is better at delivering the necessary current to your speakers than the Willsenton. As seen here, basically the entire frequency range under 1kHz is under 5 ohms. This, combined with the capabilities of the Willsenton, will likely deliver a sound may be musical but not as controlled. And if the sound coming directly out of the speaker is not that controlled, then adding a fancy Rel sub won’t tighten the sound that much.
How big is the room this system is in?