Gary Shutt shows how to get the low 7 string sound on a normal 6 string, using B standard tuning. What you need to do is get yourself some really thick and heavy gage string like 13 through 56 or a 7 string set and just not use the 9. Once you have your strings on, you tune everything down to perfect fourths. Going from the lowest (bass) to highest strings you will have: B-E-A-D-G-B.
So both of your outer strings will be B's and they will have the same lowness and chunkiness as a 7 string without the high E string. You can play your normal 6 string chords this way.