Which taps?

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.

+1 for @three_easy_payments ..  

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

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