Mystery switch


I have an old Audio Reflex receiver (Model AR-800) which has a 'DE. EMP' switch on the back: 75uS to the left and 25 uS to the right.  The 'u' actually has a preceding tail on it, so its probably a symbol. 
What does this do?
Also, it says on the back 'Do not use speakers less than 8 ohms'.  Is this as imperative as it sounds?

Thanks, 
128x128sound22card
My mid 70’s Pioneer receiver also had switchable Dolby demphasis but that was a dead end 

The power supply rails ,transformers are small that is why it cannot handle lower ohm loads .never heard of this receiver. How old is it 
I would not invest in it ,no where to modify it properly ,which you 
would need ample space on any decent aftermarket parts which are all much bigger.