Sonus Faber Finish Maintenace

I would just use a clean Swiffer duster for regular dust-removal maintenance.  The finish is likely a polyurethane, which means it's pretty tough.  I would just use a microfiber cloth with a spritz of auto detail spray, like the ones offered by Griots or Mequires.  The main thing is to wipe very gently in straight lines and don't use abrasive cloths like paper towels.  If you don't have microfiber, a soft 100% cotton cloth will also be OK, but microfiber is the best.  

I think staying AWAY from wax’s and the like is important. The SF’s are finished in a VERY high gloss...ANY ( and I do mean ANY) minor grit will mar the finish! Always use a microfiber cloth and I believe best to have it dry and obviously clean. The string grills are another question altogether??? I have never heard any recommendation that made sense on these. Frankly IMHO, the string grills are one stupid idea; although they do look good ( while they last).

I have the Amati Futura, too, red violin with the nickel chrome tops. The red violin finish is in perfect condition, shining like new. However, one of the nickel tops shows some light lines when in the sun, traces of when it was wiped. I wouldn't call it scratches, but they don't wipe off. What would be a good way to polish the nickel?

Walnut oil wipe on let sit a few hours and wipe it all off even if the finish is poly the wood under will absorb some oil and thus look better and last longer just use care around the drivers.

Thanks @johnk however I believe you are referring to the wood part. The wood is in perfect condition, I am talking about the top plate, chrome/nickel, that has some visible traces or minuscule scratches that show in direct sunlight. Some kind of polish maybe? I don't dare experiment with any random polish product.