Yes, got that, I guess set of AA batteries rather than AA battery pack is the more correct terminology. 4AA rechargeable batteries wired in series is 1.2v*4 = 4.8v. 2600 mAh is 2.6 Ah. 2.6*4.8 = 12.5 Watt-Hrs, divided by (rounding) 2.5 hours of runtime is a 5 Watt draw for the MP6.

For the external pack, 20,000 mah is probably the rating of the internal li-ion (or li-poly) cells, which are at 3.7v. The 3.7v needs to be converted to a 5v output. Assuming conservatively that that conversion is 80% efficient, you have 3.7v * 20 A-hr * 80% = 59 Watt-Hrs. Divided by 5.8 hours of runtime (5 hours, 49 minutes), you get a power draw of 10 Watts for the MP6.

Either the Mixpre-6 is very inefficient somehow when powered by the USB-C input, or the 20,000 mAh battery pack is old or wasn't fully charged. At least, so says me who is hoping for way better runtimes of a 20,000 mAh battery pack.

Sorry for all the math, just wanted to be clear what I was assuming for anyone trying to follow my calculations.