As you say, you don't need a volume control on the DAC. Your amp's volume control will do the job fine.
You don't have to start with a DAC. You can take the signal off the Mac Mini's headphone output and run it right to your amp. The better your cable, the better this will sound.
If you do use a DAC you have a 99.9% chance of getting better sound than the headphone output will give you. If it were me starting out, though, I would start cheap and upgrade only when my sonics seemed a bit less than satisfying.
You don't have to start with a DAC. You can take the signal off the Mac Mini's headphone output and run it right to your amp. The better your cable, the better this will sound.
If you do use a DAC you have a 99.9% chance of getting better sound than the headphone output will give you. If it were me starting out, though, I would start cheap and upgrade only when my sonics seemed a bit less than satisfying.