I am a bass player in addition to being a guitarist and use a 7-string bass (The F# I usually use is .170, so you can pretty much choke a rhino with it. On the high end, the high C string I use is a .34) The reason I'm telling you this is that I experience an even bigger difference in volume between my high and low strings. To balance out my sound, I am a huge believer in using compression. For bass, I might use a 4 or 5:1 ratio. I run the compression first in my chain.
For guitar, it's a bit different. Unfortunately, compression makes high levels of gain even noisier, so I only use a 2 or 3:1 ratio and liberally use the noise gate. It tightens up the lower strings, but doesn't color the sound like overdrive would (which might or might not be a good thing). The other (possible) downside to compression is that you lose some of your dynamics. If you were playing 70's style rock or rolling off the gain to clean up your sound, then this would be a bad thing. If you're going for the modern gain chugga-chugga thing, then this might be a good thing. Plus, if you are playing a lot of single-note stuff or harmonics, compression will help your shredding.
Finally, one other thing mentioned in other threads is that you can lower the bass at the preamp level and then boost it after the amp.
Hope this helps,
-AL