Jesus lol it's just a decibel, at volume that dip isn't going to even be noticable. It's even under load.
What sonic defender is trying to say is anytime you have a peak like that it's going to change how the other frequencies sound- there's no way around it. Looks like fiio went for a more gentle amplitude and topping went for a steeper wave
There's you sound difference, and of course how the shaping effects the curve of the phones or iems you're using. And what impedance, 'load'
Thankfully I've got iems with a great sub bass response. So I definitely don't need a bass boost. But I'd rather eq myself and get the exact curve I wanted. I am using balanced though and am3d.
Yes it is only 1 decibel, but at least to my ears (this is of course a subjective thing) the bass boost does not sound "right" and the NX4's sounds more natural. Plus I am not talking about 1 dB difference in say a noisy street, trying to compare an hydraulic hammer vs a diesel engine on a bus, I am talking a 1 dB listening on headphones in a quiet room, using the same headphones and music file. Hearing is very sensitive and adaptive. At the night when in bed I can clearly hear the noise floor of the room, which is pretty low. I am sure that at day in the same room I would not be able to make out all those little noise, less on a busy street.
This is of course just my perception and not a factual thing. Maybe it is just the slope in the Topping is more steep as you say.