I have not seen this before. Looks like a proprietary connection (Only that system uses it), what is the model name of the current HiFi?
You could cut the cables off the old speakers and connect them to new speakers. This will keep the plugs intact. Check that the new speakers are 8ohms (8Ω symbol on them).
6ohms is okay but I wouldn't go any lower.
The ohms is the resistance the speaker creates when current is pushed through it. If it is too easy for current to flow though the speaker then the amplifier can get damaged. Think of it like driving your car in first gear for too long, the engine is designed to have a force resisting it.
It will be fine![]()