DSP spec sheets list a lot of numbers, but a handful matter most for real-world tuning: channel count (how many independently processed outputs you get), EQ bands per channel (how finely you can shape each output's response), and SNR (how clean the processor's output is at idle).
More EQ bands per channel allow more surgical correction of narrow peaks, which matters most for car-cabin tuning where reflections create sharper, more numerous peaks than a typical home room.
Don't overlook input handling specs — a DSP's maximum input voltage and its own SNR rating affect how cleanly it processes whatever signal your head unit or factory amplifier feeds it; an excellent DSP fed a noisy or clipped input will still produce a noisy or clipped output.
When comparing two DSPs on paper, channel count and band count are the easiest specs to compare directly, but actual sound quality differences often come down to internal SNR and how the manufacturer's tuning software handles phase and time alignment — details that don't always show up as a single number on a spec sheet.