|So why do some preamps have headphone outs on them? Shouldn't they only be used to adjust equalization? Also, what is a DAC ever used for? I understand the digital to analog business, but I'm confused in application of--both, really.
I think headphone jacks are included in preamps, because it's a desired feature. I don't think it adds much to the cost to add a simple jack/amplifier connection, and you can use the input selector, gain control, power supply etc. for the headphone circuit too. Note I said it doesn't add much to add a "simple" one - a good one may be a different story. I suspect that good headphone circuits being widespread in mass consumer electronics probably stopped happening with the advent of electronic receivers in the early 80's (integrated circuits vs. discrete components). So, your old silver 25 lb. pioneer receiver probably has a better headphone circuit than your sleek black receiver you bought 10 years ago. [this is all wildly generalized, of course - there are definitely exceptions] Preamps are somewhat of a different story, since they tend to be higher end products in the first place. In fact, I have a nice preamp in storage (Bryston) that I need to dust off and try its headphone section. It could very well be exceptionally good.
RE: DAC's - not sure exactly what your question is. Most people will have a component DAC in their system if they feel that the one they have been using is a weak link in the audio chain. For example, many modern receivers/preamps have built-in DAC's for decoding the digital stream from an outboard DVD or CD player (via Coaxial RCA digital or Toslink optical). However, these on-board DAC's are not necessarily the last word in sound quality. Therefore, people add a DAC component to get better performance. The output from the DAC is in the form of RCA analog output, generally. You can take that output, and use it as input to your home stereo, your headphone amplifier etc. When technology changes in DAC's you can sell it to a person less obsessed with audio and upgrade without touching the rest of your system.
This is probably way more than you wanted to hear from me, but once I get on a roll, it's hard to stop.