First of all what amp do you have and if you have a manual does it say what signals go where on the insert jack?
Next, you say that you were able to get audio on the mic input. Did you just plug your guitar cable into the TRS adapter and then into the mic? Or did you plug the insert cable into the adapter and then to the mic input? If you plugged the insert into the mic and got audio you should be able to do the same with the line-in. The only difference is that you will only get one channel to record.
Make sure you select the line-in in the record volume control panel. Also check if you can unmute the line-in on the playback panel and hear it in your speakers. ASIO4ALL should not be required to simply get a recording.
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