Much thanks. I appreciate your insight.
I'm only tuning down to C#, so I'm staying well within guitar range, but my ears tell me something may be amiss.
I'm under the impression that so long as the lower frequencies(which a typical guitar speaker may not handle well) aren't being pushed too hard at too high a level, you should be alright. If you don't already use an EQ, maybe have one just to cut the lows down, if you notice the speaker being driven a bit too much. Or, set up a crossover for your guitar and bass cabinets, since you can never have too many cabinets.

In any event, a lot of bands play tuned to A or A#, but it's never a bad idea to be safe all the same.