I currently play a Epiphone SG G400 through a Line 6 Spider IV 15W at home and the distortion channels seem extremely muddy.
I play in a band and when I play with them I use a Horizon Cabinet and a Johnson Millennium JM250 Amp Head with a Boss DS-1 and I'm not having the same problem so I don't believe it's entirely the pick ups fault (although I'll change them eventually).
I just don't know if I should save up for a new amp altogether or run my Line 6 on clean and get a pedal (like a Boss Power Stack). Any advice would be appreciated!
Firstly, I'd try another guitar through your Spider to better gauge what's causing the muddiness. As the video below demonstrates, the Spider's distortion is pretty crisp and clear with what looks like a cheap Epiphone Les Paul Special.
It's likely that your JM250 is opening up the pickups in a way that no 15 watt combo would be able to. So it may simply be that the stock pickups in the G400 aren't as "compatible" with the Spider's distortion.
You also have to compare like with like - what does the DS-1 sound like through the Spider's clean channel?
I would personally go for a pedal, because in the long run you can also use it with your head (or any amp you might use in future) and it should cost less than a new practice amp.