Seek suggestions for mid-classic organ & orchestra

Organ concerti: Poulenc, Copland, a movement or so in Holst's The Planets, Samuel Barber has a short, but plaster cracking piece, don't remember the name (do some research). Hope this helps.

Try the Jongens Symphonie Concertante, a fairly modern but very interesting and melodic enough piece. Dorian has a fine recording (coupled with the Organ Symphony) with Gillou and the Dallas Symphony; there's a good Telarc recording of it as well with Michael Murray. Also try the Poulenc Organ Concerto (also on Telarc with Murray). And the best plaster-cracker of them all is Pomp and Pipes on Reference Recordings, arrangements for organ and brass band that'll knock your woofers out!

In my opinion, pickings are a bit slim for what you describe. Here are two you might try...
Poulenc - Concerto for Organ, String and Timpani
Poulenc - Organ Concerto

Of course, if you would like to just luxuriate in some amazing and powerful (solo) organ music, listen to the Franck "Piece Heroique" or the Vierne "Symphony 5" or the Karg-Elert "Symphony Chorale" or the Widor "Symphony 5".

For plaster-cracking, window-rattling, shake-the-fleas-off-the-dog ORGAN music, I cannot think of anything better than Max Reger's 'Introduction, Passacaglia, and Fugue' - perfect for subwoofer-testing and a great example of early 20th century organ music. Widor's Symphony 5 and Symphony 8 are also wonderful examples.

For combined organ/orchestra works, I second the Holst Planets suggestion (good brass, especially in 'Saturn'). My favorite version is the late '60s London/Decca recording of Zubin Mehta with the L.A. (one issue is a 2-desk set which also includes Also Sprach Zarathustra). Copland's 'Organ Symphony' is also a wonderful piece of music though I have not heard it enough to recommend a specific recording. Perhaps other 'goners could advise?