That be a hole notha discussion. It depends on what you have in the way of a calibration mic, etc. I'd say either the audiocontrol or Helix. If your still in the market for a 4 channel amp, then may be one of these.
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AC DM-608, the Helix Mini or the Dayton DSP-408
AC
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Helix (better value for the money than the AC)
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Dayton
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You will need a Calibration MIC for the dayton, that would be another $30-$50.
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If you have an iPhone, I think that the AC and Helix software will work with the MIC on an iPhone, if Android, will need that mic. If you go with the DAYTON, you will need a laptop and either that mic or the 3.5mm one. You will be able to to use the REW free software with the dayton, it comes with a link for the free REW software.