I have both. To be honest, I have never run the B7K flat so don't actually know how they compare. However, If there is any tone difference with the B7K flat, it will be so very slight you'll never hear it in a mix. The B7K is a huge upgrade, I sold my original B3K as I found it too restrictive tonally, the B7K is in a different league, a true preamp. Having them both together stacked is just brutal, and possible without bucket loads of hiss now thanks to the Attack Cut mode, huge improvement for both units.