When you connect two pairs of speakers to an amplifier, the combined impedance is halved. So, when two pairs of 8Ω (Ohm) speakers are connected, the overall impedance becomes 4Ω, even though the individual impedance for each speaker remains unchanged at 8Ω.
With this in mind, the EXA100 has a power output of 100W RMS into 8Ω, which is increased to 155W at 4Ω.
This quoted amplifier power rating is per channel. If you attach 2 pairs of 8Ω speakers, the overall impedance halves to 4Ω, so you'll be sending 155W per channel. This is of course then split between the two speakers attached to that channel.
It is not recommended to connect 2 pairs of 4 Ohm or 6 Ohm speakers to the EXA100, as the impedance is halved meaning the impedance will be 2 Ohms or 3 Ohms respectively. This could result in the amplifier being driven too hard and CAP5 protection being activated, or damage being done to the amplifier and/or speakers.