what kind of discount on demo speakers?

20-25% off seems reasonable.
I was once able to get 30% off because the speaker was on its way from the factory and the guy/gal canceled the order before it arrived.

I would look and see if there are the same speakers on Agon listed as demo models and see what other dealers are asking for them as demo models. Also look at used models and figure you will pay a bit of a premium over used because of the manufacturer's warranty. I don't think there are ahrd and fast rules. It all depends on how much you want them, how good of a relationship you have with the dealer selling them and how much hardball you want to play before getting them. Also, on the manufacturer's warranty, does the clock start when you purchase them or did the clock start when the dealership bought them and you are getting the remainder of the manufacturer's warranty (i.e., if they come with 5 year warranty and they are three years old, you are getting 2 more years of warranty coverage)? If the latter, obviously pay less of a premium over a used model without warranty coverage.

Demo speakers are just used speakers.

Some warranties only cover from the date sent to the dealer. Some do cover from the date the customer buys them.
Check to see just how old they really are.
The dealer certainly knows the exact date they bought them.
If they are waffling on the date. be wary.
Make certain the warranty starts the day YOU buy them. not the date the dealer bought them.

Standard used is 50% of new, with varying factors. Are they 'desirable' speakers lots of folks want? Are they cold, nobody wants them?
Then how old. Two years or more is too old and price should drop.
Are they a current model? or a discontinued model?

For five year old speakers where the warranty is from time dealer took delivery, and the speaker is a discontinued model.. 50% tops is a fair price. With damage,, ot would drop to BELOW 50%

For speakers which are hot and folks love them, the current model, and only a year old, plus the warranty starts the day YOU buy them.. 30% off is fine. With a noticable ding.. 35% off.

AND remember:
Since ANYONE can buy brand new current model speakers for 15% to 20% off.. just remember that...

I would be trying to get 20% off the top on new speakers, so I would be thinking more.
I'm sure it depends on the demand for that specific item, but
I've frequently seen much bigger discounts on demos.