I have the A40 IIs, with a Polk sub, on a 737. I absolutely adore them. However, I am noticing at higher volumes they lose some of their sweetness in the mids/higs on some tracks (though my other half can't hear anything amiss). Perhaps it's age-related (had them for about 25 years; didn't start using them as mains until a year ago), or a quirk in their crossovers that some reviewers noticed 30 years ago.
If you are paying < $60 for a pair, by all means try them out!
With just about any bookshelf with an 8" or smaller woofer, you are almost surely going to want a sub to fill-out the bass. On a 626, a powered one is the only way to go. I have the Polk PSW-10. It's a bit feisty to get right, but the payback is worth it. On other units, make sure they take speaker-level inputs because the Pios don't have outputs for subs.
I auditioned the Polk Monitor 4 and I did not like them at all on the 737. Sounded kind of hollow and unremarkable. Vocals on Rumours and early Doobies were flat; on the BAs you can almost hug each of the musicians and vocalists. However, they sounded right at home on a friend's HT. I get the feeling they simply were not designed to be used on a 2-channel setup.