I use the same headphones at work. The first thing that comes to mind is to plug them into the L/R main outputs. If you are already using these to feed an amp, you will need a splitter. I don't think you will realize an output level drop at the amp, but if you do, just bump your level. Of course, this output will vary with volume, and you may not want that.
Second is the "tape out". I've never used it on the 990, but in all my experience in the past, tape out is a constant level that does not vary with volume. This may be more appropriate for the headphones as I know that they will drop out of the signal gets too low. On my PC at work, I leave the PC volume all the way up, and vary the volume with the Sennheiser only. That also maximizes your SNR for the RF connection.
Finally, you may wish to use the "Zone 2" out if you're not using it for something else. This will give grandma the option to listen to a different program while another plays on the "main zone". She will also have a fun and small remote all to herself. The downside is that "Zone 2" only works with analog inputs, so if you are going digital from the DVD player, suppose, you will also have to connect it to the DVD's analog outs. Not that big of a deal, as I've had to "double up" connections in order for my "Zone 2" to work properly.
Those are the three that pop to mind first.
Good luck.