Thread Trying to set up a Scarlett 2i4 for recording

The picture below shows the front-panel settings on my Scarlett 2i4.

The first input is from an Alesis SR-16 drum machine, with the volume on the SR-16 set at about 75% of full. As you can see, the switch is set to "line" the gain is set at about the 60%-70% mark, and the volume is good.

The second input is from the headphone jack on a Mustang II guitar amp. Note that the input is set to "inst", the gain is turned all the way up. The master volume on the amp is set to 4. The perceived volume through the headphone monitor (note that the mix knob is set to 50/50 source-and-input) of the guitar with these settings is in the same range as that of the SR-16.

What I can't figure out is why I'm having to change the type of input to "inst" and crank the gain up fully on this input to get appropriate volume from the guitar. Note that I can plug my headphones directly into the SR-16 without changing any settings, and the volume in the headphones is about the same as I'm monitoring through the interface. If I plug the headphones directly into the headphone jack on the amp without changing any settings, the volume is again about what I'm getting out of the Direct Monitor on the interface!! It would seem that if plugging the same pair of headphones into either the SR-16 or the headphone jack on the amp is giving me acceptable volume, why am I having to change the inputs so drastically to get equal volume from the two sources?

Just for chuckles, I switched everything between the two inputs, and observe the same behavior, so it seems to be the inputs on the interface are reacting to the two sources differently, even though the signal from the two isn't noticeably different when listening to them directly just with the headphones.

What am I doing wrong here?

I'm not sure if it means anything, but the volume for both inputs is GREATLY reduced if I switch the Direct Monitor switch from stereo to mono which I don't understand either, but if I can get this to work ok with the Direct Monitor set to stereo, I don't see a problem for me with that.

Any help is greatly appreciated!

Thanks!