Using a guitar amp as a monitor is not wise. The amp and the speaker in the amp is not meant to reproduce an input signal perfectly and they don't in general. They usually have quite severe bass and treble roll-off.
If you are using a modeller into the front of a guitar amplifier, it will likely be better to switch off any cab/speaker simulations you have switched on in the modeller, as these are not needed when you have a real amp/cab/speaker to take care of the treble and bass roll-off.
If you are using the HD400 into something that does reproduce input signals perfectly e.g. recording software, PA, headphones etc then you should have the amp/cab sims on, else it'll sound VERY harsh.
If you are using a modeller into the front of a guitar amplifier, it will likely be better to switch off any cab/speaker simulations you have switched on in the modeller, as these are not needed when you have a real amp/cab/speaker to take care of the treble and bass roll-off.
If you are using the HD400 into something that does reproduce input signals perfectly e.g. recording software, PA, headphones etc then you should have the amp/cab sims on, else it'll sound VERY harsh.