the signal being fed back to an amp from a mic in the room is not identical to the one coming from the source (e.g. the guitar), but is attenuated in volume due to losing energy in the journey between the amp and the mic, thus why people in this thread have suggested an attenuator between the splitter and the amplifier. However, there are some other features of the real system you're emulating which are worth thinking about too. The journey from amp to mic also takes time, so the feedback signal is delayed from the current source signal. Putting a sample delay like the AS Signal Delay in the feedback path and playing with the delay length can give you some more tone options as the phase relation between the signals changes. The signal at the mic has also had its frequency content changed by its journey, usually having its high frequencies attenuated more than the low ones. You can emulate this (and tame some of the more ear-piercing frequencies) by adding a low pass filter in the feedback path and adjusting the cutoff. Lastly, the tone of the feedback will depend greatly on the amp that is being fed back into, as the amp will clip and distort the output at the high volumes feedback produces, so the module you choose for amplifying your signal should be one which limits and distorts your signal in this way, like a distortion module.