Since you like the sound you’re getting and are not opposed to adding a sub, I’d definitely go that route. Not only will it bring you to full range, but your whole soundstage will open up and expand and your amp may not have to work as hard depending on which sub you choose. If it were me, I’d get a Vandy 2Wq sub as they’re extremely musical and offer a unique design that works with your existing system and eases integration. There’s one available now...
https://tmraudio.com/speakers/subwoofers/vandersteen-2wq-powered-subwoofer-mahogany-finish-x-2-cross...
Hard to imagine making a bigger improvement for relatively so little money. Best of luck.
https://tmraudio.com/speakers/subwoofers/vandersteen-2wq-powered-subwoofer-mahogany-finish-x-2-cross...
Hard to imagine making a bigger improvement for relatively so little money. Best of luck.