These are certainly not hard and fast rules, just some things I have learned work for me:
Watch your lows, cut them out. I often high-pass up to 250hz or so depending on how distorted the bass is, among other factors of course.
Turn down the gain. Turn it down some more. Turn on the tubescreamer, no gain, full volume. Helps it be tight.
Use lower output pickups. For reference, anything I tune that low would have something from bare knuckles's vintage hot line. The moderns are just too hot for me, with lower tunings and thicker strings it's just mud. I prefer to get the gain from the amp. So if you need to turn that back up, that's ok. 
Speaking of thicker strings...at a 25.5" scale, a .074" string tuned to G# will give you the same tension that a .046 tuned to standard E will. So go thick. I use a 74 on my 27.7" scale in A# and I love the tension.
We can tolerate less and less dissonance the lower we tune. Make sure intonation is perfect, there's not much room for error this low. Everything will stick out.
Buy a baritone. Unless you have a bari 7. Drop B is about it for me on a standard scale. Come to the dark side....
For whatever it's worth, open chords wont sound in tune below ~A, and there's not much to do about it. This was my experience and when I asked David Bendeth about thinking I was missing something he pretty much said "Nope, it won't work, tune up or buy a bass". 
Now I stay in A# and use Transpose for anything lower. Sounds better to me than any guitar that's actually set up that low.
Lastly, throwing a compressor on there may actually blow up your low end, not control it. High gain amps don't really need to be compressed further, but if you want to try it, I would use one with a high-pass filter so that you're not bring the low end up when you increase the makeup gain. I would forget about this. Many, if not most heavy rock/metal albums do not compress the guitars. It's very helpful on cleans, but not so much for high-gain.
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