My amp is a Marshall Valvestate 100 (combo with 1 valve) and I would like to record the guitar with my audio interface, though without using mic. My idea is to plug the guitar to Marshall, than disconnect the speaker (112) and plug a jack cable there (into the port where you normally plug Marshall speaker's jack), then plug the other tip of that cable into my audio interface's instrument line (Behringer UMC22) so I can monitor the sound on my computer speakers. And I am wondering if this idea is going to ruin my amp or the audio interface?
You can not connect the head's speaker output to an interface without potentially damaging either the head or the interface.
If you want to get a signal out of your amp and into an audio interface without using a microphone, you have a few ways to do it. The sound you get will be different than what you normally hear from the amp as the speaker helps create the tone that your amp produces.
You could use a D.I Box that is specifically designed to handle speaker output. You would plug the speaker out into the D.I., pass through to the speaker, and take the D.I. out of the box to your interface. The tone from doing this us usually quite pinched sounding.
Many amp heads will have Effects Send/receive jacks. You can use the effects send jack to get output, usually from the pre-amp section of your amp.
If the amp has a D.I. or Line out, this is designed to send your amp's signal out to other equipment and is usually sending the signal that is after the Effects loop.
The headphone out will usually work, but the sound quality will likely be poor or noisy as the headphone amp is usually a separate amp circuit, and not really designed for audio output other than for headphones.
The Valvestate 100 actually has a really useful feature: on the back panel it has a D.I. Jack. This is specified for:
connection to recording or P.A. equipment, featuring Marshall’s acclaimed Speaker Emulation circuitry as featured on the JTM60
So this will not only give you a line out, but it will also deliver a tone that sounds like the amp itself, so when you hook it up to your audio interface you will be able to record that lovely Marshall tone.
Do not disconnect the speaker and try to connect the speaker output to your audio interface. You will destroy the audio interface!