Thanks for your responses!

SO I was afraid of opening it, but when I did I found this. What are those messy things and are they cheap to replace? Also the jack looks weird.

Thanks for the response, I tried it and it didn't work. Then connected the preamp out to another speaker to check if the jacks worked properly and I got the same scratchy noise from rolling the volume knob, but it seemed to have a louder output, and the speaker was smaller...
At last I tried plugging the guitar to input 2 and the noise was gone! The volume problem is still there though, the amp isn't as loud as other smaller amps i've tried. 

Hi, I'm new here, and I'm trying to know what the problem is with this amp I bought. The previous owner replaced the speaker and did a general manteinance job on the amp. When I brought it home it seemed  fine, but then when set the volume to 10 it isn't as loud as it should. It seems as if even my brother's Pathfinder 15r is louder...

I installed a killswitch on my SG and after that it all sounded very noisey, with lots of feedback on high gain settings. There's also a scratchy noise when I roll the volume pot. I thought I messed up the wiring on the guitar, but trying with other guitars does the same, and rolling the volume knob on my guitar plugged on another amp works fine.

I think I soldered the switch with the guitar plugged in, could that have messed up something on the amp? Thanks in advance, I hope to fix it soon, its such a lovely amp.