MIDI does not carry audio data, only MIDI messages. The output you are connecting from iRig Stomp I/O to your amp is audio out from the audio interface of iRig Stomp I/O. So you'd need to have iRig Stomp I/O connected via USB to your computer and send the output(s) on it to your amplifier. The amplifier will play the sound from iRig Stomp I/O's audio interface, and should not have anything to do with a Bluetooth connection from your amp to your computer (since that also introduces latency but not likely the latency you are experiencing here). To reduce the latency you are hearing, while using iRig Stomp I/O as your audio interface you need to find the lowest buffer size possible in your setup which will provide the lowest latency while not causing any audio breakups/pops/crackles.
The use of iRig Stomp I/O as an audio interface, including it's onboard I/O and how to connect to various types of devices/speakers/etc should be well-covered in the product manual available in your IK User Area or on the iRig Stomp I/O product page.
If none of this helps, I would recommend seeking official direct IK Support via
http://www.ikmultimedia.com/contact-support so the team can assist you in getting iRig Stomp I/O working flawlessly.
