Be sure you do your research. There's a thread called "Darkglass disappointment" with an ongoing discussion of a guy that grabbed a B3K and it's not what he was looking for. The Sansamp is going to give you that SVT overdriven tube style of tone. The Darkglass is going to give you more of a distortion, but some of us are using the B7K as a preamp with the drive pretty low. The blend and level pedals of the Darkglass are different than other pedals. You can get a variety of tones with it but it takes some playing with it to find what you're looking for.
If you're interested more in a light grit, Darkglass makes a Vintage Microtubes that's more of the tone that the Sansamp has.
In a nutshell, someone on the other thread put it best with their description of "For an Alice in Chains tone, Vintage Microtubes or Sansamp, for Pantera tone - B3K/B7K.
I have a Sansamp RBI rack unit, and B7K. My tone used to be the Sansamp RBI on all the time. Then I got the B7K and now my tone is the B7K always on with the blend and level at 50%, and the Drive at around 9 o'clock. It does sparkle the mids a bit, but I'm always bumping my mids up in my band setting.
Also, keep in mind my experience with the Sansamp is with the Rack unit, which has a Mid control on it. I don't believe any of the pedal units have a mid. I started with the BDDI, but then when I built my rack I upgraded to the RBI rack mounted Sansamp.
I did some testing with my B7K a while back to our drummer's track of one of our songs. I can't remember where the drive is set in the recording but I'm pretty sure it's either 75% or maxed. Maybe it'll help you decide if you want that Sansamp grit/overdrive, or distortion.
And for the record, I'm actually looking to get a DG VMT to add to my B7K in the near future. Because I find myself wanting them both.
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