Go with the 50 watt. It won't hurt your ears as bad. And it will save you some money when re-tubing. I have a 100 watt master volume and I can barely turn it on. Whereas my two two 60 watt amps are no problem. In some ways most amps get their distortion from the preamp section. The power tubes...
Use only EL34/6CA7 power tubes and ECC83/12AX7 preamp tubes for that amp. My favorites are JJ EL34 and JJ ECC83S. The new Svetlana EL34 and Tung-Sol EL34 sound good too. I don't trust Chinese EL34s myself. If you want to boycott Russian products go with the JJs. Remember the amp needs to have...
It could be a bad power tube. Maybe even a bad electrolytic capacitor. Or something else. Tell the tech the symptoms. By the way what brand of power tubes do you have in your amp?