I have a tube amp question?????

You certainly aren't going to rock the house with this amp, but if that was your interest--you probably wouldn't be looking at this amp anyway. If you are in a moderate sized room and listen reasonably close (less than 10 feet--I don't know if your room or speakers are conducive to that setup), you will probably be so engaged by the tube amp, that I doubt you will be looking for more volume. The only other aspect I would consider is the ohm (loading) of the speaker. I know you said 6 ohms, but that's nominal. You might want to be sure that it doesn't dip to some very low impedance in certain instances. That can create some difficulties for some tube (and other) amps.

I once owned a 30 watt tube amp that I used to drive Vandersteen 2c's (88db) in a 12 by 26 room. It occaisionally clipped and distorted so I would think that 12 watts would not be enough, however this is only my opinion.

I doubt that 12 watts into the given speaker load you described would be very satisfying. If I couldn't audition I wouldn't do it.

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.