550a - the hardware difference.
I A/B compared this with the Waves plug. The low frequencies showed similarities, and the plug seemed like a cleaner version of the hardware. From 300hz and up, the API 550a proved warmer and rounder than the comparatively flat Waves plugin. No source revealed this better than some dirty electric guitars. At matched settings, the Waves plug exhibited a bright, clear sound. The hardware 550a transformed the guitar into a warmer, more harmonically rich track that sat in the mix while remaining present. The guitar was still punchy, but softer around the harsh edges where the plug left it harsh by comparison.
As for the build quality, I don't think it could be more solid, and the design is actually very, very aesthetically impressive in person.
I gave it a 4.5 instead of 5 because the top is a little strident. That's the only complaint.
Preferred uses: Everything except lead vocals, but it absolutely owns overdriven electric guitars.
I could see the plug handing the hardware's job on a kick drum in a busy mix, but that's about it. If you like the plugs, the hardware is going to blow your mind.