Which taps?

You read first and make sure the amp will be all right. You have a manual? Find one. I tap what sound the best. Then I check for heat. Smoke is to late.. :-)

Most amps if you screw up they will pop a fuse if the load is to heavy. 

I like using an 8 ohm tap for 6-12 ohms. But I use the same type of drivers. Small planars, and ribbons.  Easy loads. I can't get a class D to heat up. An AB valves amp runs a lot cooler behind very efficient light driver material. Nothing is heavy about planars or ribbons. Magnets?

Your bouncing around a bit. Get a heat gun, it will tell you a lot.. Check the heat index. Rule of thumb, cooler is better, and listen..

Regards

Try all taps.  The ones that sound the best to you are the right ones. 

+1 for @three_easy_payments ..  

I looked into this when choosing a new to me integrated tube amp for my vintage 16 ohm speakers.

General rule: use the tap closest to the 'nominal' impedance of the speakers, thus use the 8 ohm taps for your 15 ohm speakers.

It will be fine, a slight shift in frequency response will occur as the speaker's crossover was designed for 16 ohm taps. you may or may not hear the difference, as I said, slight.

Guitarists sometimes use different impedances purposely to get a 'different' 'preferred' sound for the specific song(s) or in the specific space they are playing.

I bought my version of Cayin A88T because it has 16 ohm taps as well as 8 and 4. I could have accepted the slight change, then the newer models with 4 and 8 (no 16) have external bias adjusters and meters.

Ralph (atmasphere) stated that my Cayin will have less distortion at it's 16 ohm taps. I remember that, but not the explanation.