It is completely illegal to void warranties that are purchased through unauthorized dealers.
There is a law called the Magnuson-Moss warranty act. Its fairly lengthy, but here is an piece:
The Act prohibits anyone who offers a written warranty from disclaiming or modifying implied warranties. This means that no matter how broad or narrow your written warranty is, your customers always will receive the basic protection of the implied warranty of merchantability.
It goes on to specifically mention "tie-in's". Basically stating a manufacturer cant void a product because you didnt use "genuine GM parts" (example)
Or that you didnt buy it from a specific company.
I have NEVER sued anyone, but you better bet if Marantz said "Oh, you didnt buy from an authorized dealer, you're S.O.L", I'd get a lawyer
Sorry, but it makes me mad!!