Randy.......Having been an observer to the inauguration of your two channel system and watched and read about it's expansion and changes I think it is accurate to say you will ultimately be unhappy with a mismatched subwoofer arrangement. Again, my observations have lead me to believe you tend to be detail oriented. This leads me to believe the mismatch will eventually bug you, perhaps not from a performance standpoint as much as from an aesthetic one.
My second thought about your post is not all subwoofers are boomy, nor do you have to be a bass-head to appreciate a properly integrated bottom octave in your system. As clean as the Music Culture RL21's sound, the 6.5" midrange/woofer does not reach the frequency depths a good subwoofer does in stride. Even though the RL21's midrange woofer's claimed range is 29Hz, it is substantially rolled off at that point. What's even more important is the inter-modulation distortion generated in the midrange as the driver attempts to reproduce the lowest frequency range that exceeds its capabilities.
I had a similar experience with the Sonus faber Guarneri Memento speakers that I had in my studio system. On their own they sounded very good, even down into the mid 45Hz territory where they began to roll off quickly below that range. When I added a pair of powered McIntosh XLS112 subwoofers and a Bryston 10B active crossover the performance difference was night and day. By allowing the Bryston crosover to route everything below 70Hz to the subwoofers, thus relieving the Guarneri Memento's of having to deal with the lowest octave, the midrange performance and dynamic range of the GM's improved substantially, with the added benefit of now having a full range performance that included rich deep bass. The advantage of this arrangement proved to be a genuine step up in the system's sound across the board. You can read more about my experience here:
Bryston 10B Active Crossover In Studio System
I said that to make this suggestion. JL Audio has recently introduce a new E-Series of subwoofers. They are reasonably priced and remarkably sophisticated in the different ways they can be incorporated into a system. I have a friend who uses a single
JL Audio E112 subwoofer with two way JBL speakers. The E112 allows the low frequency crossover point to send only the frequency range above that point to the power amplifier driving the speakers. In other words, no external crossover is necessary to keep the low frequencies at the subwoofer and everything above the crossover point routed to the left and right channel speakers. This capability can be used with a single subwoofer or a stereo pair. This arrangement would be perfect for your system. The improved midrange performance of the RL20's will astonish you, not to mention the additional dynamic range available from your power amplifier because it no longer has to reproduce the lowest octave, and the E112 subwoofer will reproduce the lowest octave with the clarity and speed you seek.
Yes, I heard you when you said you didn't want to consider a new purchase. I'm just planting a seed for you to think about. The E-Series JL Audio subwoofers are reasonably priced and are serious performers, and that includes their E110, too.