Different headphones cause different input levels / latency

I'm trying two different headphones with mixed results. One is a pair of AKG's - full-sized over-the-ear studio headphones, which cause a lot of latency. The others are Bose 5QB7Z, which are more like earbuds.

There are drastic differences in input level when using these two headphones. With the AKG's in, the levels are good. With the Bose, they are much lower, which means I have to raise the input level in the app. This creates a lot of noise in the recording.

I'm wondering why the difference in levels?

I'm using an iPad Air2. Loopy HD is up to date.

thanks