Best speakers for $300/pr

$300 can go a very long way on the used market.  It depends on his listening preferences (rock, classical, jazz), room size, how powerful the Marantz is and if he wants monitors or floor standing speakers.  Too many variations to start mentioning brands at this point until the above is known.  

Once the above is known then you can tailor his needs towards a certain sound from speaker manufacturers.  British 2 way monitors have a certain sound to them, sweet and disappearing, that I tend to like very much but may not be for everyone depending on their musical tastes.  

Please keep us posted,
Lou

The problem is you get used to a certain sound and even if you hear something superior, you don't always realize it when you hear it.  Having been in this hobby for 30 years, I believe we all have an aural memory such as I've described above.  That is why when I make any change in my system, I like to live with the different gear so that the new sound grows on me.