I talked to the guys at Funky Junk (london) the other day about mics, and got recomended the Beyer Ribbon (sorry, don't know the number), or the Coles Ribbon mic.
The Beyer is dead cheap (about £120), and the Coles, which is a beautiful handmade british thingy that is used widely by the BBC and Abbey Road studios is about £600. My mate took the Coles and Beyer for a test on trumpet, and ended up buying the Coles. The Beyer is more 'toppy' and sensitive. We didn't test it on piano, but the recomendations both for the Beyer and Coles were strong! It's a cheap option anyway. Haven't tried the Octava Ribbon, but I've heard they're excellent value!
Have not done a piano yet since I just got in a pair of Gefell M300 mics but I've been told they are between the sound of the KM84 and 184. They run less in price than the 184 also. If you have the piano available most of the time order a pair on trial and exchange them if you do not like them.
I must admit the pair I have are still in their evaluation stage because I've only tried them on acoustic guitar. So far they sound like a keeper there but I've not used them there enough yet.
Read DOT's reply to me here [="http://www.studioforums.com/6/ubb.x?a=tpc&s=1436041581&f=1526095781&m=1126036424"]Studio Forums[/] He does a lot of mic testing.
Some people have also reported using good boundary mics on piano. I'd probably not buy boundry mics just to try there but they may be a consideration if you work remote.
2 At4050´s inside the instrument, one facing mids to low, the other obviously mid to high. A Third At4050 shall be used outside to pick up an overall room sound, if it is decent enough. ALso, you can use a "center" mic close to the tail.