I've got an Aprilaire 550 on my furnace, but it basically never hits the humidity target. I know it's on (I can see microbubbles moving through the tube between the solenoid and the sponge), but there's no water coming down the drain tube, and the sponge is basically dry. Should there be excess water? Should the sponge be wet to the touch? There's a strainer and a restrictor in the plastic tube after the solenoid - I'm tempted to widen the hole in it, but not sure what's the expected behavior.
You can take the line off after the solenoid and clean out the orfice carefully (do not enlarge the hole) and the strainer. Shut off the water supply, remove the tubing to the solenoid, place the end in a bucket and flush out the tubing. Reconnect the supply tubing, turn on the water, get a catch basin or cup to catch the water and turn on the humidifier for a few seconds to insure that the solenoid is opening and not plugged. Reassemble the solenoid discharge tubing, and anything else that you had to take apart. Turn on the dehumidifier' Water should run into the humidifier distribution tray and drip onto and thru the filter pad. The excess water will drain out the bottom of the humidifier through the tubing and into the floor drain. If you have a manual that came with the humidifier read it first to get familiar with the layout and parts you will be cleaning. The service company should inspect and service this humidifier yearly.