Ok, I listened to the video and this is an easy one that in fact is covered in the owner's manual too.
First, there is a difference between drive and drive VOLUME.
With a higher output bass, with the drive volume. The DRIVE control adjusts the amount of GAIN within the overdrive engine, and in the lower gears (specifically 1 and 2), since there is so much available gain and the compression compliance is high (ratio low), it's possible to drive the O/D engine into clipping which is a completely different mechanism than overdrive. That's what the low sensitivity switch position is for, to accommodate higher output basses at high drive gain settings.
The drive VOLUME adjusts the level OUT of the overdrive engine, and it's used along with the clean volume to balance the two signal paths. When using the lower gears, it's a lot easier to hear the overdriven signal tone and texture with much lower amounts of clean signal.
I have attached a copy of the owner's manual, please read through it because there is a lot of information that will be helpful to addressing this.