Matt I don't have the Avantone but something very similar. A pretty decent 3 or 4" driver in small box and it sounds great. I think as long as the speaker itself has a decent magnet and can handle a bit of power than anything is going to work pretty well. The concept is all the same. Mine is passive and I drive it from a small Hi Fi amp I have lying around. It should not have a wide frequency response in fact. I still might invest in an Avantone though, they look good and apparently are perfect for this job. Many many studios in the past had small speakers like this. You still see Auratones on ebay as well.
Tracey if you are running KRK's and they do tend to be a bit mid range lacking then all the more reason for the small monitor I have recommended. Because that is all midrange and the exact opposite to your main speakers so it would be a great addition.
Also here is the reason why
one is so much better. If you have two small speakers even you can still pan stuff in order to separate things like two similar sounding guitar parts and the panning is a slack way of creating separation. But with only one speaker the two guitar parts end up directly behind each other in the speaker and you have to use other more clever ways to separate them such as recording different guitars, amps and treating the two sounds much more differently in the mix. As they did with Beatles mono mixes for example. If they layered rhythm guitar parts and they did, they went to a lot of trouble to alter the guitar sounds each time in order to separate those parts in a mono speaker.