Get the Arcam CD73 because it will allow you to extract as much sound performance from your existing Red Book CD collection regardless of whether you decide to purchase the Sennheiser HD650 down the road or build a high fidelity stereo system. Also, let me tell you a story. I have both the Grado SR-60 headphones and Ultimate Ears Ue-10 PRO earphones. I also have the Delphi Roady2 XM Satellite Radio tuner and Tivoli Audio Henry Kloss Model One AM/FM Radio plus a highly modified ModWright LLC Pioneer Elite DV-59AVi univeral DVD-Video player that plays both DVD-Audio and SA-CD. When I plug my Grado SR-60s into the headphone jack located in the back of my radio, the sound quality is pretty good, but it is nowhere near high fidelity in any sense of the word. When I plug in my Ue-10 PROs, the sound quality improves, but it still does not sound like naturally occuring sound that I am accustomed to hearing from my reference universal source component.
I am now listening to Sinead O'Connors Throw Down Your Arms Red Book CD with my Ue-10 PRO which is hooked up to my reference system (please see my user profile as I have a lot of equipment). The sound is glorious. It is so smooth, detailed, articulate and effortless that I have enjoyed this CD and every type of disc that I play through my reference grade universal source component. I took the time to listen to the same music earlier today through my Grado SR-60s and the headphones sounded tremendously more musical and enjoyable not because they are top of the line Grado headphones (that would be the discontinued Grado Professional Series 1), but my highly modified source component which cost me $2800.00 USD can make music sound musical.
So, that is my story. Do not be mistaken in thinking that the source component is a trivial component in a high fidelity or high end AV system. Invest in the the right source component for your budget and it will repay you with endless hours of musical delight while still permitting you to experiment with different headphones or earphones and headphone amplifiers when you acquire more funds.
I hope my story is helpful. I am sticking to my own advice by investing in a much higher grade source component, the Denon DVD 5910 at $3800.00 USD because I am convinced that the music (or film) always starts at the source.