please try to think about it , what happens if you got a nice sine wave , and you start to attenuate back & forth , totally out of sync!! Thats right, a CLICKS& POPS ! Inside there is resistor ladder, like in other digital pots resolution is limited. Try to read those digipot threads, and you get whats the problem. You have to spend effort to fight off crosstalk. This layout offers itself for crosstalk big time . Typical mixer application is something like - overdriving the signal adding second and other harmonics, PGA is totally transparent, its usually employed as preamp before ampllification.