Hi Jack (no pun ?)
This is a complicated question, but there are some guidelines I can give, without going deeply into impedance matching and tube data (well it depends...)
As far as the amp is concerned (consider yourself the amp and you are happy when all is well and experience pain when something is wrong) the size of the driver makes no difference electrically.
The impedance (4/8/16ohm) does.
First rule: Tube amps do not mind a dead short on the speaker output, but hate open circuit. (kind of opposite to solid state amps). So electronically speaking all 4 ohm speakers are OK (in the short/medium term).
See: your 15watter probably has 2 EL84's in push-pull mode.
From Duncan's Amp Pages
http://tdsl.duncanamps.com/show.php?des=EL84 I get this data (here we go... put on your thinking cap)
AB1 P/P 300 300 72-92 8-22 130 shared 8,000 17 10
AB1 P/P 300 300 -14.7 15-92 1.6-22 8,000 17 10
Look at the application data: If you want 15 (~17) watts, you'll need a 8000 output transformer. If this transformer is rolled/designed for a 8 ohm output - that is a fixed thing, you cannot change the transformer's impedance (unless it has multiple taps and a selector switch - which I doubt on your 15watter)
So: 8000ohms into 8ohm. If you connect a 4 ohm speaker (which is half of 8 ohm) the load on the tubes drop to 4000 ohms (half of 8000). Makes sense ??? ... and if you connect a 16 ohm speaker the tubes are loaded with 16000 ohms.
When tubes that are designed to be loaded at 8000 ohms work into a 4000 ohm load, they emit more electrons, and the oxides on the anode lasts shorter than expected. Earlier tube failure = money gone :'(
But... if you are ZZ Top and you want to lay down a track in the studio, and the amp only sings in the 4000 ohm setting, then you go for it and switch back later to 8 ohm speaker. But if you are Pietie who plays his amp for 20 years without a service, stick with the correct suggested speaker impedance. (usually this info is in the user manual)
