Speaker Impedance Demystified

Rear Panel Impedance Switch

Selects the impedance connected to the Speaker Output jacks.  This setting should be matched to the connected load.  If using multiple cabinets, they should be each of the same impedance.  See Table 1 for a list of common configurations and the total impedance that should be selected.

Table 1 – Speaker Cabinet Impedance

Total Impedance (Z) Number of Cabinets Cabinet Impedance (Z) Note
4 Ω 1 4 Ω Single 4 Ω load.
8 Ω 1 8 Ω Single 8 Ω load.
16 Ω 1 16 Ω Single 16 Ω load.
2 Ω 2 4 Ω Dual 4 Ω loads.  Unsafe!
4 Ω 2 8 Ω Dual 8 Ω loads.
8 Ω 2 16 Ω Dual 16 Ω loads.
1 Ω 4 4 Ω Quad 4 Ω loads.  Unsafe!
2 Ω 4 8 Ω Quad 8 Ω loads.  Unsafe!
4 Ω 4 16 Ω Quad 16 Ω loads.
TODO — starting point below based on email conversation:
The main thing with impedance is matching.  Always run the amp on the correct impedance for the given load.  If running more than one cab it’s always best to combine only like cabinets — such as 2x 16 ohm cabs for 8 ohms total, 2x 8 ohm cabs for 4 ohms total, 4x 16 ohm cabs for 4 ohms total, etc.  So long as things are always matched, you’ll have the optimal power output and it will sound the best.  Mismatching can cause weird highs, weak lows, etc.
The best way to envision impedance matching is to consider a freight train pushing a feather — you have this huge power that moves relatively slow, pushing on something that effortless floats through the air.  Or, picture you trying to push a car, you might be able to get it moving, but not very fast.
Low impedance microphones vs. high work the same way.  Pro mics are low impedance so they can drive hundreds of feet of cable without loss.  Guitars are historically high impedance, which is why we have all sorts of problems with high-frequency loss from long/cheap cables and such.  Active pickups, like EMGs, have a low-impedance output which solves all those high-freq loss issues.  A buffer at the beginning of your board will help this too.  But, I digress.  🙂
To actually answer your question — after things are matched, it comes down only to the speakers themselves.  The output transformer in the amp, if designed properly, won’t cause any change in sound between output impedances, if it’s driving a matched load.  There can be changes in the sonic characteristics of the speakers though.  You may notice an 8 ohm V30 doesn’t sound quite as bright as a 16 ohm, for example.  This comes down to the voice coil and how it responds differently.  Unfortunately, Celestion only publishes 8 ohm frequency response data, but I bet comparing to a 16 ohm it’s a bit different.