That's a really good price for the A5000, if it's in good condition. Since it's missing the microphone, it's likely missing several other pieces. Dod you get a remote control with it?
The Yamaha is likely not going to have bass/midbass that is as full sounding as the Marantz. However, it is going to have better high frequency resolution. It could come across a little bright/harsh, especially if you pair it with the Monoprice Monolith amp and your B&W speakers. You'll have to let us know how it works out. There are ways to help calm down the Yamaha if your sound is too bright/harsh (i.e. fuse upgrade and power cord).
The Yamaha is likely not going to have bass/midbass that is as full sounding as the Marantz. However, it is going to have better high frequency resolution. It could come across a little bright/harsh, especially if you pair it with the Monoprice Monolith amp and your B&W speakers. You'll have to let us know how it works out. There are ways to help calm down the Yamaha if your sound is too bright/harsh (i.e. fuse upgrade and power cord).