I finally had to take the plunge with Speakons when I got my Shuttle too.
What I found: it seems the NL-4 (4 pin) is the more common configuration, even for 2 conductor applications, though the NL-2 PLUG will plug into either 2 pin or 4 pin jacks.

(from Neutrik, click bigger)
With the NL-4 they just use 2 of the pins (+1/-1 for the Shuttle) & leave the +2/-2 unused.
From the GB manual:
SPEAKER OUTPUTS – The SHUTTLE 6.0® provides two Neutrik Speak-On™ connectors (wired 1+/1-). The speaker jacks are paralleled. The minimum speaker load is 4 ohms. Do not ground either the “+” or the “-“ outputs.
Also from the GB manual:
The Shuttle 6.0® is equipped with a ¼” unbalanced instrument input, input stage clip detector, mute switch with LED indicator, 12AX7 tube preamp with extended range variable input gain, 3 switched signal shape filters with LED indicators, 3 band active equalization with sweepable midrange frequency, master volume control, proprietary output limiter, ¼” tuner output, aux input, effects loop, balanced direct out with ground lift/pre-post/mic-line switches, headphone output, and
2 Neutrik NL-4 “Speak-On”™ speaker jacks. ( oops, I see you already had that part)