Modern Symphonic Composers

Sibelius, Elgar (though are these slightly earlier than you're looking for?).

Roger Sessions, Einojuhani Rutavaara, Lutoslawski, Alfred Schnittke. Penderecki.

Obviously I can't remember (or spell) everyone's first name!

The later symphonies (No.4 and beyond) by Rautavaara are very well constructed and enjoyable to listen; he was searching for his own voice in his earlier works.  Rautavaara's protege, Kalevi Aho, is another renowned symphonist and, thanks to the advocacy of the label BIS, is very famous amongst audiophilic circles (although I haven't listened to his works).  Also from Northern Europe we have Eduard Tubin, whose symphonies sound like modernized Nielsen.

The "even number" symphonies by Alfred Schnittke are quite important listens; the "odd number" ones are wild-cards, and has too much pastiches for my liking.

Hans Werner Henze has written 10 symphonies so far, and the first 6 are conveniently packaged in a 2-CD set by Deutsche Grammophon.

At least two British composers have contributed considerably to the modern symphonic literature: The four symphonies by Michael Tippett are to me his best works, far and above his incomprehensible operas.   The 11 symphonies by Robert Simpson are likewise highly renowned.

In sheer number, no one beats Alan Hovhaness: his symphonies numbers to the hundreds (he destroyed a lot of them, yet 67 remains).  Of course not everything got recorded.  Many people think his works are trite; I happen to love them and want to get my hands on everything.

There are a few other composers I can highly recommend:

Valentin Silvestrov is very high on my list of modern, yet very accessible, composers. Dark, brooding music and the 5th Symphony is not to be missed.

Vagn Holmboe wrote a series of symphonies that are quite extraordinary (get the Bis set).

Rued Laangard wrote a lot of "symphonies" and there's not a dull moment in any of them. Some are quite brief, but all are well worth your time. (Da Capo)

If you like really weird, stunningly atmospheric, creepy music you have to get the symphonies of Humphrey Searle. If you know the soundtrack to the original 1960's horror movie The Haunting, you'll know what to expect.

If lighter fare is your game, the three symphonies of Nino Rota are highly recommended, too.

You could investigate Alla Pavlova - a Ukrainian/Russian who now lives in the US.  She writes in a very accessible Rachmaninovian/film music style.  That sounds a little more dismissive than it's meant - I enjoy her work a lot.