Hey cool, I just bought a vox vt30 and yeah it does get a little soft, when earphones are plugged in.
But hey, if you notice somethig, the presets are set to be just nice when headphones are plugged in. So if you plug your headphone in, and use any preset, the volume would be just nice(power level to 15watt).
The digital sound is caused by well, the amp. It is afterall a modelling amp.
Anyway, if the volume is too soft, maybe try adjusting the gain. If you read the manual that comes with it, it does illustrate that the sound goes through the gain first and passes the power level last. Maybe adjusting the power level would help too cause mine can fill up a 2 storey terrace house with volume max at 15 watt.