I pulled 2 tubes.
I have the PR266 and it stated it in the manual. If I remember correctly you pull out the inner two power tubes. There are a lot of tubes but the power tubes are larger than the others and there are only four of them. There are two metal clips on each side of each tube. You press them down and wiggle the tube free.
You then need to change the imepence which is pretty easy to do also. It's just a switch on the back of the amp. Some models, like mine, also have a power cut switch as well. I am now running at a theoretical 15 watts. It's a very loud 15 watts but much more friendly than 100. I have also hear you can disconnect one of the speakers as well and all these things are easily reversable when you have a gig.
I would recommend googling for the manual of your particular model. Any specifics I give you might be wrong for the Twin you have.