Using a "Y" adaptor, such as this one
, will not degrade sonics all that much, if at all.
All you need to do is connect the two male connectors to the pre-out and amp in connectors on the Marantz. The cable going to the Khatrago can then be plugged into the female connector.
If having an extra connector inline bothers you, a longer "Y" cable can be made with a short run between the in and out of the Marantz and a longer leg to the Khartago with a male connector.
When you get the preamp, you will still need to run a cable to the Marantz and turn off the Khartago if you want to listen to headphones only.