The best replacement for head unit is amp.

The best alternatives to head unit are rated as following:
★★★★★  amp
★★  sound processor
★★  amp crossover
★  amplifier
★  pioneer
  line out converter
  alpine
  kenwood
  pioneer head unit

Your alternative for head unit is not listed? Suggest!
Speaking about head unit, one probably shouldn't forget about aux input.

Other search terms related to head unit and their ratings are:
★★★★★ aux input
★★★★ amp
★★★ steering wheel controls
★★★ aux
★ amplifier
You have in mind a companion for head unit and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of head unit in all years. Click on graph for more.
head unit
amp
sound processor