Matching Correct Cab Impedance/Wiring with Amp Head

What kind of speakers do you have? The only speakers that I know of that actually switch from 4 to 8 ohms are the Epifani DIST series.

Since your amp is 4 ohms minimum, it depends upon whether you will use one cab or both. If you are going to use both cabs it is essential that you set them to 8 ohms. Two 8 ohm cabs in a parallel will provide the minimum 4 ohm load your amp is rated for. If the cabs are set to 4 ohms, the load will be 2 ohms which could potentially damage your amp.

When you use one cab, you can make a decision based upon the power rating of the speaker, power rating of the amp at 4 and 8 ohms and the resultant sound. The amp will make maximum power at 4 ohms. If the cabs RMS rating at 4 ohms is equal to or greater than the amps rating at 4 ohms, you may want to use the 4 ohm setting if you need full volume.

However, something to keep in mind is many speakers are excursion limited below their RMS rating for low frequencies. This is the mechanical power rating of the system versus the RMS rating, which is a thermal rating. Few companies actually publish any data related to their cab's mechanical power rating; so it's wise to use caution, especially if you really like pumping up the lows.

If you don't need the full volume of the amp, consider setting the speaker to 8 ohms so it limits the amp's output so there is less chance of damaging a speaker by accident. The different impedance may have some subtle effect on sound quality as well.

 

I believe theres some Hartke cabs with a 4/8 ohm switch.