Are you European? The European dongle has like 1/2 the power as the US dongle.Perhaps you need more amplification or a better seal? I guess it’s subjective to some extent, but I think these have amazingly good bass (especially given the price)! I will admit I haven’t tried a ton of IEMs - AirPods Pro, TripleFi 10 Pro, UE900, Fiio FD5.
Edit: hold on, I just tried the Apple dongle at max volume with the Zeros playing Dire Straits Brothers in Arms, and it isn’t even close to what I would consider reference level. Am I nuts or does my dongle suck?