I would suggest that you take a step back and examine your whole system before investing a big chunk into another subwoofer. You stated that your system is for "stereo music". If it is just for music, just be aware that there isn't much deep bass in most music. Pipe organ music certainly does. And some electronic music. But, most truly deep bass resides in the movie world. If you do want deeper extension for movies, then yes, you should look for something that'll go deeper then the Ultracube.
However, just consider that this thread has been discussing subwoofers that are 2 or 3 times the price of your mains! Probably 90% of music is coming from the mains, so if you want a well-balanced system, wouldn't you want your mains to be at least of the same quality as your subwoofer? If you're happy with the Monitor 7's (they are pretty decent speakers for their price), I would suggest that you at least
consider looking for another Ultracube to even out the in-room frequency response and give you some more headroom.
Otherwise, I'd be quite hesitant to invest a substantial amount into a new, more capable subwoofer, before looking at my mains. Unless, of course, that's your long-term plan. If so, forget everything I said...
Oh, one last thing! If you, or someone you know, has even modest woodworking skills and access to a workshop, you can bang out an excellent subwoofer for half the cost of a commercial product.
