Gravitational wave (GW) contribution from the loudest binary versus the first multipole in frequency bins at f = 1.98 nHz (left) and f = 27.68 nHz (right). The darker and lighter shaded contours indicate regions containing 68% and 95% of the realisations, respectively. Colours correspond to the same models as in Fig. 1. The star marks the median of (Ω_tot, C₁/C₀). The dashed grey vertical and horizontal lines denote the current NG15 upper bound on C₁/C₀ (Agazie et al. 2023e) and the individual SMBH binary detection (Agazie et al. 2023a), respectively.