Ideally it should be of the same timbre for seamless transition and panning of sound from front to rear so that means getting the same line/brand/model series like that of your fronts fortunately most manufacturers have a wall mount model specifically for rears. For most HT enthusiasts they opt to choose other brands due to availability, speaker design, cost, cosmetics etc. and then they try their best to match it using DSP or any signal manipulation derivative from the AVR but for the discerning enthusiast its a non negotiable to get the same line, model series and brand.
Underneath the couch? That would be a waste. There are many websites that will suggest the optimal placement for rear surrounds