Input ground loop hum only when p10 cables or XLR without phantom power are connected

Actually, pro-audio equipment (such as the Scarlett) typically have floating ground on their internal circuits.  This requires special cable wiring on certain conditions such as a TRS-to-RCA cable where you actually have to disconnect the ground/shield wire from the RCA cable itself.  See this link:

http://www.rane.com/note110.html

Many pre-made cables are not wired correctly here.  I actually had to modify a Mogami RCA-to-TRS cable as well as a custom XLR-to-TRS cable to remove a ground loop hum issue when I used a Rane MLM82a stereo line mixer between my computer/dac and some powered monitors.

Just re-read through your configuration description. The inputs on the Scarlett 2i4 are balanced, but you might try switching the input switch to "INST" below the input. Otherwise, you might try building the Cable #14 (make sure the mono plug is connected to the guitar/bass and the balanced 3-pin plug is connected to Scarlett -- you essentially have to disconnect the groud/shield wire on the balanced plug).

One quick thing you could try here is to disconnect the power cord from your laptop and try running a recording session using the laptop battery power only. If this removes the ground-loop hum, you might need a usb galvanic isolation adapter between your laptop and the Scarlett.