I'd be inclined to get a better bass first - there are a lot of inexpensive basses which will be a step up from the G4M one. Does the amp have a headphone output? If so, try using that rather than the speaker and see if it gives an improvement. The amp upgrade is going to depend on what you intend to do - if you're just going to be playing at home, you won't need huge amounts of power, and portability won't be so much of an issue, whereas if you want to go gigging with it, you'll want a fair amount of poke and you will probably want reasonably lightweight gear. For the time being, as others have suggested, a Nux Mighty Plug or Boss Katana Go, or a Zoom B1 or B3 or similar would give you a better listening experience with the bonus of being able to play with effects.