Can headphone outs Sony C555ES replace a preamp?

I was dumb enough to sell my old preamp before I bought a new one. Since I only use one source (my CD player), I used a Musical Fidelity headphone amp as my preamp. Honestly, it sounded great. Anyway, I recently plugged my amplifiers directly into the (volume controlled) headphone outputs of my Sony C555ES and it sounds pretty darn good. Is there anyone out there that can tell me why a preamp would sound better than a direct link between the source and the amplifiers? By they way, the output from the CD players is more than adequate. Anything beyond the 10:00 position is too loud for me.