You could probably accomplish what you wanted with a pair of No.25 subs from REL, depending on room size and how loud you like to playback your music.
The response for REL regarding subwoofers
The response from REL regarding adding a REL Subwoofer for Cornerhorns.
"Unfortunately a speaker with that kind of sensitivity is going to be near impossible for any subwoofer to match with. The 2 volumes will never be able to match well. "
Is that true? It doesn't sound true to me. The SPL to power will be steeper than a less efficient speaker but it should jut be a matter of adjusting a powered subs gain curve to match that of the high efficiency speaker output or just using a more sensitive driver in the sub.
To bad I really wanted to buy a REL sub but I guess I'm not understanding how speakers work.