I tried it with and without the speaker level converter with two aftermarket radios on two '03s with moon & tunes. While the aftermarket radio did work without the speaker level converter, a level of white noise was introduced that I found was unacceptable. Without the converter, it may not be too noticeable when listening to a louder song, but any quiet passages will have a hiss. Not to mention things were very loud in general, even at lower volume levels.
I never noticed any pops when not using the converter, just static... I recall the pops being prevalent when people tried to wire in the aftermarket radio's line level outputs into the speaker wiring in the harness as a workaround for this issue.
The speaker level converter adds an extra cost, more points of failure and more junk you have to find space for inside the dash, but to me it's worth it. Your mileage may vary.
03 Vibe base. Born 10/14/2002 06:07 AM
Neptune. Auto, Moon & Tunes, power package. 129k mi.
'13 Sonic LT - 1.4L ECOTEC turbo/6A. 84k mi.
'15 Escape SE - 1.6L EcoBoost, FWD. 41k mi. (hers)