Speaker noise - hissing

Yes. Most probably the small tubes. If the hiss is annoying you will need to get 'low noise' tubes. They are usually a bit (or a lot) more expensive so be careful not to buy 'more' than you actually need. Also consider that changing tubes, unless you stick with the same brand, might well changes its sound a bit (or a lot).

More info is needed.

What make and model for the amp? How old is it? Has it been serviced lately? How many hours are on the tubes?

Does the hiss occur when only the amp is on and no sources are connected?

Same with the speakers. If the speakers are particularly efficient you may notice hiss that a less efficient set of speakers would not reveal.

In short, the more information provided, the better the answer.

I've found that Sovteks and a lot of common Russian-made tubes can be quite noisy, especially the Sovtek 12AX7LPS. It most definitely is always the preamp tubes or the smaller tube types such as 12AX7/12AU7/12AT7, 5751, etc.. Never had noise problems with Output tubes.

Welcome to the musical world of tubes. In exchange for SS nasties, there is tube hiss...our imperfect world. The EH and Tungsol tubes are known to be very quiet. Get phono grade tubes if you want really quiet. I find the tubes at RAM/Rodger Modjesky, Tube Depot, and Tubestore have good quality. There are others, but I have purchased from these. jallen

Are you referring to a 50 watt power amp or integrated? You didn't mention whether you have a tubed preamp which would be the first source I would investigate.