It's only a 30W amplifier! Overall performance is going to be somewhat limited.
Connecting different kinds of speakers in series can produce strange frequency responses as their impedances are not constant over the audio band. E.g. a 6Ω speaker isn't really 6Ω, that's just an average for the purpose of amplifier matching. Its impedance might vary from 4 to 20Ω at different frequencies, but it won't vary in the same way as a different model. Therefore it's always best to connect different speakers in parallel across the amplifier output which has a low impedance.
My suggestion would be:
3 x 3Ω in series to make 9Ω
2 x 6Ω in series to make 12Ω
Connect those two strings in parallel to make 5.1Ω
Connect to 4Ω output.
The power delivered to each 3Ω speaker will be 3/4 of the power delivered to each 6Ω speaker which is 1.2dB difference and not very important. Their efficiencies and power ratings are likely to be much more different than this.
Not quite all of the amplifier power will be available into the 5.1Ω load but it will not be damaged or overloaded. The stated load impedances on the outputs are optimums, any variation higher or lower will decrease the available power but too much lower can damage the amplifier.
Don't forget that putting an 8Ω load on the 8Ω output and a 4Ω load on the 4Ω output at the same time will overload the amplifier. You could connect 16Ω load to the 8Ω output and 8Ω load to the 4Ω output simultaneously and get full power correctly matched.
PA amplifiers like this use an output transformer, which is how they are able to accommodate a range of different load impedances and drive 100V line from a 12V battery. The 16Ω output winding is tapped at 50% and 71% to give the 4Ω and 8Ω taps. The transformer is also the reason they have poor low-frequency response (this one only goes down to 100Hz, to save size, weight and cost) and high distortion.
tell me..
