The reason for this is that we want the amp to max out constantly with very low volume.
It's no good for a live band unless your amp is micd thru the PA system. The amp is very low level volume.
For learning about building tube amps, you should get a kit first to become familiar w/ building. It's like all the parts in one place.
And saves a lot of parts orders.
But be careful not to electrocute yourself please.
Many harp players use 14-30 Watts, or even more. The trick is the microphone, which does not cause squealing feedback. To find a good harp mic may be expensive. There is high isolation, to reduce the feedback problems, Or multiband EQ is very handy indeed.
Without a good harp mic, you will not get the result you are looking for.