You're confusing yourself bro
A DAC, is a Digital-to-Analog-Converter. What a DAC does is that it processes the digital signal from your computer or phone into analog signals which can then be turned into sound by your headphone's drivers.
An amp, is an Amplifier, designed to amplify the current analog signal sent, giving more power to power hungry headphones with high impedance or low sensitivity.
There is no "which one is better", only "which model is better", but even so the latter question is hard to answer sometimes.
Do you need an amp? Not really, but you might benefit from it, especially paired with the HD598s.
Do you need a DAC? Most likely, especially if you are currently listening to music just through your phone or your computer's motherboard (no dedicated soundcard)
Should you get the Q1? Maybe, but honestly at this price nothing can go wrong. As for the other models, I have not heard them and cannot vouch for them.