When I A/B'd the GB Shuttle 6.0 and the Ampeg PF, it was the Shuttle hands down for me. The Shuttle also is rated as having a little more power, and I like its tube pre. The Shuttle also sounded better to my ears than Markbass, GK, and SWR micro/Class D amps. As a final note, I should add that when I started shopping I was fairly set on getting an Ampeg PF-500, and was even disappointed when it didn't pan out in my comparisons.