The best way to deal with this is to use the Caustic or AudioPocket editor and identify some of the longer factory samples (1 second or longer) and target those to be replaced.

The best way I can describe it is that the Keys is a more "optimistic" device, whereas the Bass is better at is specific stated task.

I wish it was a better way to upload samples into it other than using an iPhone or aftermarket app

There are 100 sample slots for storage, but whatever you upload needs to be in place of something else, so you can free up more storage that way.

This is a great way to get those digital drum machine sounds and one-offs like sirens into your Volca Rig