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.
4-channel car amplifier with digital signal processing. AccuBASS circuitry for improved bass response. variable high- and low-pass filters (20-20,000 Hz at 12 or 24 dB/octave). optional AC-BT25 Bluetooth adapter available.
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)
The smallest HELIX signal processor gets a worthy successor with the DSP MINI MK2. A powerful 64 Bit audio DSP and BurrBrown converters combined with 96 kHz sampling rate and high resolution audio bandwidth up to over 40 kHz provide exceptional sound quality.
Dayton
Dayton Audio DSP-408 4x8 DSP Digital Signal Processor for Home and Car AudioTake Control of Your Sound Ease of use has always had its place in the consumer electronics industry. Dayton Audio has now brought to market a digital signal processor (DSP) that combines versatility, incredible sound...
You will need a Calibration MIC for the dayton, that would be another $30-$50.
Dayton Audio iMM-6C Calibrated USB-C Measurement Microphone for Apple/ AndroidDayton Audio IMM-6C USB-C Measurement Microphone for Tablets IPhone IPad and AndroidTurn any phone or tablet with a USB-C jack into a laboratory grade audio measurement device with Dayton Audio's iMM-6C USB-C...
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.