United we stand, divided we....stand?

Q: Should I buy a standalone headphone amplifier + a standalone DAC or an amp + DAC combination unit?

A: If you buy a amp+dac combo unit (a single enclosure holds both the ehadphone amplifier as well as the DAC) and think you will be using the amp alone or the DAC alone at some point, then the key is to buy a combo that allows DAC output as line out without involving the amplifier circuit (i.e. the DAC output from the combo can bypass the amp) so that you don't end up double amping. So for example if you have an amp/dac combo and want to buy a better standalone amp, you don't want to run the signal through two amplifier circuits in tandem, in this case the amp in the amp/DAC and then the standalone amp, since you will introduce additional distortion into the signal.

Some combos (e.g. the Audioengine D1) will not allow you to decouple the DAC output from the amp which makes them inferior as standalone DACs. If you are planning to own multiple amps and/or DACs, getting separate units (such as the O2 and ODAC individually) as opposed to a combo (such as the O2 and ODAC in a single unit) makes much more sense because you can swap out the DAC or amp to something different.

However if you have decided that your purchase will be endgame 9at least for a while) then getting a single unit makes more sense because it is more convenient + less clutter.