I currently play a Dingwall NG2 (37" fanned scale) tuned to double drop D, with Kalium strings (around 210 gauge for the low D string), and it sounds beastly; dark and heavy as all get out. Throw a bit of clanky overdrive like some Darkglass or Tech21 VT bass or Oxford, etc. and you will cut right through. No way am I going to let the guitarist tune lower than me hahaha!
If you want to tune that low, go for it, and don't let the misinformed tell you that it can't be done, or that you can't hear notes that low!!! It works quite well. The key is using a High Pass Filter to cut out the subsonic muck that your speakers can't reproduce, and will waste your amp's power. Also EQ is extremely important. Boost the mids to bring out the harmonics, and you will be heard! DOn't over boost the bass or it will just be a boomy mess.
As several people above have said, you need a long scale (37") otherwise you will not be getting the kind of note definition or string tension you want for it to be anywhere near playable. If you can't invest in a bass with a longer scale, or you would simply prefer to not tune that low, I would follow the advice of others above and just roll with the B standard - For both E minor and E major, B is your fifth, so throw that in when your guitarists are hammering away on the open E. You don't always have to match the same notes as the guitarists are using when they are riffing, playing at a 5th or 3rd can really thicken up an already heavy part, and add some extra texture and interest.
Cheers, and have fun!