I'd have to agree many of the presets are terrible. I'm not sure cheap headphones are the problem, the sound is equally awful on my HD25II's & AKG K702's.
The sounds I've built from scratch are awesome, so no complaints from me & I've only used a fraction of what's available.
The problem I have with tweaking the presets is that there are so many combinations of gear I'm not familiar with, that I'm not always sure where root of the problem lies. It's easier to start with an amp/cab combination & then add effects one at a time. That way it's easier to judge the impact each individual effect has in conjuction with the others. I get that people who are familiar & experienced with tweaking modelling effects, or maybe even own some actual gear being modelled won't need to do this. But they're probably looking at something a little higher up the food chain than the Pod go.