I found that even with a sub the bx5's were lacking in low-mid definition. With the 8" monitors I have now I still use the same sub, but only for 80hz and lower frequencies, a range that the monitors claim to handle but do so poorly IMO. I mix and produce a lot of bass heavy music so it's important I hear those frequencies.
So in comparison, I found the 5" drivers too small and the overall sound from the bx5's to be too empty (?), but with the larger (and better) monitors the stereo field (stage) is much larger and the definition brighter. For $200 the bx5's are good, but for $500+ there's larger solutions that blow them out of the water. If you don't have a choice, then by all means get a used sub and figure out how to get them working together (watch for phase issues), otherwise spend the coin to get the sound you likely desire.
I found my
Samson Resolv A8 monitors on sale at Newegg as a refurb for $250 shipped for the pair. It was a bit of a gamble. They turned out to be like new, may have just been a return. Sound great, even for $500/pair normal cost. Just an example of what to look for. There's a lot of smaller brands making monitors now, and are likely using similar internal parts, speakers, and even enclosures. I compared these with the couple 8" monitors Guitar Center had (I think Yamaha and M-Audio) and the Samson sound as good, if not better, than most there.