Yamaha HS50M for mono mixing?

Right now I only have one set of monitors (KRK 5) which I've had for over five years. I have seen it strongly recommended that it's good to have a second set of monitors so you can compare your mixes in both (if it sounds good in both then the chances of it working in well "in the real world" go up substantially - is the argument) ... and I've also heard that mixing in mono is a good thing to do (either sometimes or a lot of the time... depending on the recommender) ... Anyway I was thinking it might be a decent idea to sort of kill two birds with one stone (and save me $200) if I were to buy just one HS50M (which I think is what I've settled on for my second set of monitors) and use that for mono mixing. Then if I can get my mix to sound good in both stereo on my KRK 5s and in mono on the HS50M (I would imagine this would be quite a bit more challenging...) then I'd could be confident that I've got everything just right. Before pulling the trigger on this plan I figured I'd ask the board here if this sounds reasonable? ~~~ so the thread doesn't get derailed I've already invested quite a bit on the acoustic response in this room ... although I'm sure it could be improved - but yeah I am aware of the importance of room acoustics and I've read a lot about that and spent a lot of money over at GIK Acoustics.

if you want mono and cheap then this looks a cool project to try. it's a DIY version of the auratone, I am keen to give it a go some time but I have an similar old bose speaker from out of a movie projection room that does the trick quite nicely :)

Mike Senior recommends a Mixcube, or I believe a Fostex 6301 which should be cheaper ($180?) for a single speaker. (Actually the on-line resources for his book have other alternatives, including I believe the same DIY link mentioned by Sonic Vampire.)