How much discount typically for a new MC275 to expect from a dealer?

Not sure if it works with Macintosh but some dealers (for other restricted brands) will provide a generous trade in allowance as a sort of non-discount discount, if you have something to trade.  

Your fears of buying used are unfounded. There are plenty of 275's of very recent vintage for sale here regularly by reputable members. Take a deep breath and buy one.  

There is no new Mc Intosh available now. It is replica and sound is not as good as vintage one.

I ALWAYS like to buy a demo piece when available. You still get the Warrantee, which you occasionally need to use. But that can happen with sealed-in-box components. The most common issue could be used speakers with very small cosmetic flaws. If there's a few small marks made by speaker cables in the back, I'm not concerned. Or there's a small tear in the grills, but the dealer will replace one or both at no charge. A tube amp could be bench-tested for 3-5 days under a load,
and if it meets ALL of its specs, who's to say its "used"?
I can't comment on vintage tube amplifiers sounding better, but Night-and-day or just "different"?  

I can't tell you the name of the dealer here. They have always given me 10% and no sales tax because the state they are in is tax free. If you buy 2 pieces, you will get 15% on the second one.