@Allegro maestoso,
What
@Mark Bajkowski stated is likely possible as there are always possible issues with electronic parts, especially if something has been dropped but from the sounds of things, it is more than likely just the [I/]'normal'[/I] hiss tubes make.
Electro Harmonix is the CTH stock tube & well known for being noisy to some degree though this will also depend on head gear used, eg impedence & sensitvity, how loud you listen as well as how quiet the media you are listening to is.
In general, depending on circuit design & implementation, tube amps noise output can vary from mains point to mains point too which might explain why you hear more noise now on your slightly more sensitive headphones.
Things to note about tube amps & tubes in general :
1. No matter the tube & or amp, regardless of cost, they will all make some noise or hiss, as that is the nature of tubes, there is no silent tube but there are definitely ways the tube noise can be reduced.
2. The tube type is an obvious primary factor as some tubes & some tube brands are more noisy than others, eg, Mullard, Genelax, Jan, JJ Electronics, Tung Sol, Electro Harmonix, etc
3. Noisy tubes can also have high gain &/or microphonics (eg, that bell like sound when a tube amp is tapped, regardless whether the amp is on or not & head gear &/or speakers are plugged in)
4. Some tube suppliers can also adjust Tube Gain, Microphonics, Noise as per customer request.
5. Tubes which are known to be noisy are generally also more commonly used in guitar amps as those tubes tend to be easily available as well as affordable to a degree so I suggest searching for tubes which have low noise, low gain (unless you want that) & low microphonics.
6. As stated above, electronics & other components used as well as design implementation plays a significant part in tube amp design or any electronics for that matter, though this doesn't mean a fairly quiet amp, tube or solid, has to be ridiculously expensive just because.
But some manufacturers would like you believe this is true.
There are ways to reduce general [I/]'normal'[/I] hiss from tubes though it will depend on how far you want to take things to reduce that hiss.
1. Have a good powerboard &/or mains outlet with good EMI/RF protection &/mitagation is perhaps the most basic step one can take.
2. Try a less noisy tube.
3. Change your cables &/or power supply to see if this makes a difference though this will depend on your budget & how much you believe in tuning sound.
Hope you find this useful, feel free to ask more if you need.
Hope you have a great day !