nomad is correct, you may pull two of the power tubes (preferably the inner pair) and also you MUST also pully one rectifier tube - This will bring your Tverb to 50 watts.
And again like nomad said, you must halve the impedence of the cab output you are using. 16ohm cab gets plugged into 8ohm, 8 to 4, etc.
One thing to know through all of this is that you are only dropping the actual volume by 3db, which is not much of a difference at all. I have found through a lot of testing that dropping to 50 only really decreases the dynamics and feel of the amp while not doing much volume reduction. You also lose quite a bit of clean headroom. I would reccomend turning on the FX loop and using the loop active master to dertermine your volume while keeping the channel masters low - you can hit the sweet spot at a lower volume. Also, to get a little extra gain boost, crank the send knob to 100% (considering you are not plugging in any effects)