I just got a Q 2626. First off I think Don Boomer's answer is inaccurate since the Audiobox 96 has a mixer knob and like you, having one of those first, I was curious why this much more expensive product lacked that feature.
But I have to believe it's by the very nature of the Quantum using Thunderbolt connection, which is 5 times faster than the other, that it doesn't need that function. In other words, the AB96 with longer latency, takes advantage of the mixing knob when recording so you don't hear a delay while listening out of the speakers confusing the player. Per the AB manual:
- Mixer control (blends the input signal with the computer playback stream for zero-latency monitoring)