The Marantz you own has more than adequate power from the headphone amplifier section of the receiver to drive a pair of HD-600s well. The HD-580 is very similar in impedance and sensitivity to the HD-600, so it should work very well.
If you prefer the sound of Grado headphones to Sennheiser phones, no amplifier is going to make you change your mind. Whichever you decide to get, you will always wonder what the other headphones would sound like when listening to a certain piece of music, so why not keep both your Grados and the Senns. Both will sound very good through a Marantz receiver.
You might want to unplug the receiver, remove the cover, and blow out any dust from inside the chassis and controls using some of that compressed gas such as Dust Off. Then get some contact cleaner lube spray (available at Radio Shack (RS) and spray a moderate amount into each switch and control, working them by hand until they are turning smoothly again. You may find that your dead input is the result of a stuck tape monitor button, and the cleaner spray will free this up.
Replacing any burnt out lamps is a little more effort but definitely something you can do. If you are going to use the FM radio of the receiver, by all means take the unit to a repair shop that does quality repairs, for an FM alignment. This requires an FM signal generator and an oscilloscope as well as specific alignment procedures for that series of tuner/receiver, so this is beyond the ability of most do-it-your-self repairs.