Most headphone amps are freestanding and have a volume control. I don't know if there any benefit in running the signal through a pre and a headphone amp, prolly not a good idea. The problem you face is figuring out how to get your CD player out into both your 2 channel system and your freestanding headphone amp, not the jack out, the real signal out.
Then the usual questions regarding budget, stationary vs mobile, and tube vs SS, resistance and so on. You might want to look over some of the forum at Headfi.com It can be very helpful just don't get overwhelmed. They cover the headphone world well.
A quick suggestion that won't break the bank- I was quite pleased with a little sigle tube hybrid tube input section/pre and an SS power out. The company is "Bravo" I have the model 2. When I bought it through MassDrop it cost about $60- They are for sale on Ebay I think but the price maybe $80 at this point. The tube is a common pre amp tube the 12AU7.
Another website and small manufacturer that is hot right now is called Garage 1217 dot com. I have one of theirs now also "Project Ember" it's for tube rollers, they have other models that may be right for you. They are friendly and quick to return e-mails
