The subs are what determine what load (ohms) you're running at, it's not determined by the amp. You have to choose an amp that will put out the power you want at the load that you intend it to run, for example, if you have a 4 ohm sub rated at 200W RMS, then choose an amp that will put out 200W RMS at 4 ohms, if you have 2 of those 4 ohm subs, choose an amp that will put out 400W RMS at 2 ohm. That being said, for sound quality, it's better to run an amp at a higher impedance because the amp has more headroom, less distortion, less heat, etc.