Y does my technics sug88 say low imp beside the a and b speakers

To keep you from plugging in a high impdance (more than 8 ohms) speaker.this may damage the output transistors or module.if it says 8 ohms, do 8 ohms....if it says 16 ohms, do 16 ohm speakers.

Because you've done something with the speaker connections that's too low impedance for the amp.

Amps need a certain level of resistance from the speakers otherwise there's a risk of the amp blowing itself up. Impedance is the name for resistance that varies with audio frequency. If the speakers you connected are any good, then there'll be label on the back telling you their impedance in Ohms.

When running a single set of speakers only then the amp would prefer an 8 Ohm load but it will accept a 4 Ohm load. If you plan to run speakers in both the A and B sockets, then both sets of speakers have to be 8 Ohms and no less. It also tells you this on the printing directly above the speaker sockets on the amp.

The amp is trying to protect itself from an overload situation where it would blow its output transistors and possibly kill your speakers too.