Thanks for the link to gearslutz.com, Timrhu. Asking there is a good idea.
Martykl's suggestion to try the QSC KSub sounds very reasonable, but I'm wondering --- since my K8 speakers start to sound thin and muddled below 440 Hz, and the KSub has a frequency range (-10 dB) of 44-148 Hz, doesn't that mean adding that sub (or any sub) won't improve the 150-440 Hz range by much? If so, then I need to try something else.
Could EQing help improve that low-mid range? Or do I need to return/sell the K8s and explore other options (like the ones suggested above)?