A lot of modern amps and preamps sound great when you're jamming by yourself, but don't hold up in a band situation. The sound isn't dense enough, and the lows and highs tend to get soaked up by the bass and cymbals.
I think the best riffs and the best songs come when you're jamming and having a good time.
You know how in every heist movie they get past the security cameras that show the hallway leading to the diamonds by jamming the screens with a fake signal of everything looking safe and quiet? Usually a guard coughs so they don't notice the blip from switching to the bogus feed.
I sing both in my shower and in my car, mostly in my car, because I have this weird thing - whenever I'm singing to the radio - my friends kind of hate it - but I pick out the harmonies in my head, and I'm singing the harmonies to the tracks and I'm jamming it out.
There is no substitute for jamming and getting to know each other on the road.