Depends on synergy here, for example Broadway is highly recommended for Abyss Diana V2 headphones, as it was voiced for it. So from pure power output perspective Hugo TT2 should be enough, but it you audition them side by side, you may prefer one over the other.XI Audio Broadway - 1,5W/32Ohm, no data for 50 Ohm loads, but it would be significantly less
Regarding power output for higher resistance, it it not always true that an amp outputs more at 32 Ohm than at 50 Ohm. Depends on the design and power curve (look at Violectric HPA V281 for example). But I generally agree with you on that it is safe to assume if the manufacturer did not specify the power output on a higher load, it is safe to assume it will be less.