I have a tube amp question?????

Here's a possibility. Get your hands on a scope or someone with one. Hook up any kind of amp or receiver that is greater than 12 watts. Now power is equal to Volts squared over Resistance. The amp was probably rated at 8 ohms. This computes to about 10 Volts RMS which is about 14 volts peak. That's 14 plus and 14 minus. Hook up the scope to the speaker terminals and see how often you exceed 14 volts while playing music at a level that is satisfactory.

My guess is that you won't be happy.

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.