Yeah, an HDMI audio extractor may in fact be the best choice here. The Topping doesn't have an analog input that one could have used to make the headphone output work with the right cable.
BTW, aren't Focal Clears a bit notorious for this driver crackling / bottoming out? I would estimate the Denon to be able to output about 2.5 Vrms into these undistorted, which should be plenty given that they are rated 104 dB/1 mW (117 dB/1 V). I mean, it's still a pretty shit output due to its astronomic output impedance (and the Focals have major impedance deviation which should result in a substantial bass boost, clearly not helping matters in the crackle department), but still.