How can I match some new speakers to the ones I already have?

I’m thinking of getting some speakers to fill out home theater this Christmas. I already have a fairly nice pair of NHT speakers that I picked up years ago (mid sized, maybe 10“W x 15“H x 15“D cabinet).

I have a very nice Denon Receiver. I’m looking to do a 5.1 setup. What’s the best way to get the rest? Is it pointless to try and use these speakers for my front or rear pair?

Acoustically, my space isn’t really suited for an audiophile setup, so no use for 7.1. As always, I’m looking for a good, clean sound. No need for extreme wattage or anything.