Ohm ratings for guitar amp / extension cab

Question

I have a guitar amp that recommends the following for connecting to an external cabinet:

Only use extension cabinets with an impedance of 8 Ohms or more (e.g. 16 Ohms). NEVER use a lower impedance (e.g. 4 Ohms) or damage could result. WARNING: If you use an extension cabinet with an impedance of more than 8 Ohms your Pathfinder 15/15R’s output power will be reduced – it only produces its full output of 15 Watts when presented with an 8 Ohm speaker load.

I have been offered an extension cabinet described as having "Impedance - Switchable 4/16 Ohms mono or 2 x 8 Ohms stereo." The cabinet has two inputs labelled 'input' and 'link'.

Please could someone tell me:

1) Can the speakers in the cabinet be setup such that the cabinet will be 8 ohms running off the one input (my amp)?

2) If the above is not possible and the cabinet is 16 ohms, will I lose a lot of power by running the 8 ohm amp into it?

Thanks

Answer 1

1. Yes, but it's not really practical. You can use a 2:1 transformer to drive the speakers in parallel (i.e. 4 Ω mode), but such an extra transformer is unwieldy and may well colour the sound substantially^†.
2. You will lose 50% power. That may sound like a lot, but this is actually only a 3 dB reduction – if you've ever used a mixing console you'll know that it's actually a rather subtle change. (This is due to the way our ears perceive loudness logarithmically, and because power is quadratic in amplitude.) So, you may still be fine. 15 W is of course not that much in the first place, but even 7 W can be pretty loud, at least if you allow the amp to go into full distortion.

There's a third option that may be best for you: simply use only one of the cabinet's speakers (i.e. in stereo mode, but with only one side connected). This has then the optimal 8 Ω impedance, so it will actually be louder than if you use both speakers in series.

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^†_{There's theoretically also the option of using the cabinet in 4 Ω mode, but in series with another 4 Ω power resistor. But that gives the same volume as the 16 Ω solution, while generating more waste heat...}