I have a tube amp question?????

12 watts is to small for an 87db speaker unless you are using it for light background music.

I use 8 watts of 300B on 92db speakers with good results. Not a wall shaker but I don't like real loud music. The same 8 watts is not enough for a 90db speaker.

15 watts will do 90db quite well.

Hi, Tireguy, there is a guideline for power:
the efficiency of your speaker is 87 db means with
1 watt input, they will produce 87 db sound if you
measure sound pressure on axis, 1 meter away from
your speakers. If you want to increase 3 db, you
need to double the power of your amp. For example,
you need 2 watts to get 90db, 4 watts to get 93db
and so on.
I hope this will help you a little.

I've run across a rule of thumb that you want around 102 db. So 10 watts will get your 87 db to 97db. 20 gets you to 100db. 30 watts should have a good chance of working out.

oops, I forgot your speakers are not 8 ohms.