Not sure I follow...in the title of your post you ask if they make pre-amps with headphone jacks...there are a few...yes. Procheezy covered that...there's other stuff out there just like that.
But then you ask if a good mixer has one too...answer is yes...most mixers do have headphone jacks.
Then, you mentioned something about hearing the beat through your computer. This is where I gotta ask could you put together a complete description of what you're trying to do.
To me it's still a bit vague. The computer put a whole new spin on things. See, in the back of my mind it seemed like your questions were asking if you could record to the PC while listening to your recording and playing a backing track at the same time. We need to know more about the PC and the sound card you're using.
I would hate to see the well intentioned people here giving you advice to go out and buy something and then you find out that it won't do what you were expecting.