The AT813Rs are probably the surprise of the bunch. One of them languished at the bottom of my gigging mic box for about 20 years without much use - I used it to mic acoustic guitars should the need arise, but more recently found it was a useful vocal mic - so when I saw one on eBay recently I nabbed it for £21, on the basis that it's a useful, versatile tool and that pairs are much more useful than one mic on its own. This was despite its relatively high noise, but that doesn't matter a lot if your instrument is as loud as as melodeon, and you're fairly close to the mic (ie, you're not recording a concert from the back of the hall),
I'd be prepared to bet that some of the other AT 813/31 models are available cheaply and that a pair might be a low cost way to get into recording melodeon. The current AT model based on the same body and head shell is proper spendy, however - Thomann has the AT8031 listed at a little less than £200/mic.
I would also say that using monitor speakers playing on Youtube, these mics sound much more similar to each other than they did on my monitor headphones listening to the recordings in Audacity.
Using the headphones the C4s were my favourites. There's /something/ about them that appeals to me, though what it is I don't know. It may just be a coloration I happen to like.
G