Hello everyone, I have a question about the wiring of my cabinet.

A month ago I bought a second hand Marshall head with a Hughes & Kettner Attax 400W cab. when I got home and tried it, nothing happened, apparently the mono input of the amp was broken, and the guy I bought it from told me to just plug it into the stereo jack. Last week my amp started to act weird so I took a look inside the cab to see what was going on in there, and the previous owner had done some modification in it, the wires from the broken mono input were not connected (deliberately, there was tape around them). Since I don't know alot about wiring and impedance etc. I made a schedule to show how the wiring is like:

I'm sorry if it isn't really clear, i'm wondering if this is a healthy setup for the amp. There is sound the way it's hooked up now, but the volume is very low (it's a TSL60 and at 10 I barely pass a drummers volume) and the sound is very thin. I think it's probably the tubes but I want to make sure it isn't caused by something silly as a cab.

Thanks in advance