I was in a similar position. I heard 10 seconds of Nolly Getgood using the B7K and my mind was made up. The B7K is a phenomenal preamp. It sounds NOTHING like a SansAmp. The distortion and EQ are different and have a different impact on your tone. If you rely on the SansAmp for your general tone, but are looking for some grittier and punchier distortion, then keep one of the Sans and order the B3. If you aren't in love with the SVT emulation and want to reshape your tone, than the B7K is a good choice.
ZWKoch, S.A.S.S. #124; Fuzzrocious #150; Club Verellen #20; Five String Bass Club #558, Wisconsin Bassist #137; Club Microtubes #7; fEARful #91.