It's in your manual:
The front panel input sockets are Neutrik Combo®, which accept either an XLR male connector (you will probably have one on the end of your microphone cable) or a 1/4 (6.35 mm) jack plug. Note the Scarlett 2i2 has no Mic/lineswitch – the Focusrite preamplifier stage is automatically configured for a microphone when you plug an XLR into the input, and for a line or instrument when you connect a jack plug. Set the LINE/INST switch next to the socket to INST if you are connecting a musical instrument (a guitar in the example) via an ordinary 2-pole (TS) guitar jack, or to LINE if you are connecting a line level source such as the balanced output of a stage piano via a 3-pole (TRS) jack. Note the Combo connector accepts both TRS and TS types of jack plug.
Like I said if its line level XLR to TRS, switch it to line. TRS input on the interface cuts the level. XLR does not and will prob be too much signal for a mic input /pre amp. Your amp is already bringing your inst level up to line level so you don't need to preamp it again with the interface pre amp.
Most DI boxes take inst level high impedance and convert it to mic level low impedance. DI outputs on amps can be either line level or mic level depending on the amp.
Do some googling and look up:
It will help you understand better when configuring equip to ensure gear is compatible with each other.
MegaBass head, SWR 210, Trace 115, Ibanez RB999, 27" Imac I7, Onyx 1640I, Motu Ultralite
Last edited by uhdinator : 02-18-2013 at 08:37 AM.