I was told not to use 2 pairs of speakers with an integrated amp and it would burn the amp up I have a Yamaha 100W amp with outputs for 4 speakers.
What model? Some receivers/integrated amps and power amps with two pairs of speaker terminals could be set to A+B, but the manual and the back panel instructed users to make sure the speaker impedance was 8 to 16 Ohms for each pair. Others, like the Pioneer SX-780, showed "Speakers A,B- 4 Ohms, or more/speaker and A+B- 8 Ohms or more/speaker".
The easy way to find out if A and B are in series or parallel- connect one pair to Speakers A and select A+B. If the sound stops, the switch connects them in series but if A+B allows the sound to continue, they're in parallel.
A+B was mainly for using two different pairs, either because they sound different or so a second pair could be used in another room but observing the correct impedance does allow using two pairs in one room.
However, that was then, this is now- speaker impedance was usually more stable and 8 Ohms, not frequently 4 Ohms and difficult loads, as they often are in new speakers.
