you are correct in thinking that, Alot of heads have the option of impedance outputs, my marshall is either 4, 8 or 16 ohm
This is from my manual,
15. SPEAKER Outputs
There are 5 speaker outputs available on the
rear panel. They are labelled according to the
intended impedances:
16 ohms: connect any 16 ohms guitar cabinet to this
jack.
8 ohms: connect a single 8 ohms guitar cabinet or
two 16 ohms guitar cabinets.
4 ohms: connect a single 4 ohms guitar cabinet or
two 8 ohms guitar cabinets.
WARNING, although the JVM amplifier has 5
speaker outputs never attempt to connect more
speakers than rated. The safe combinations are
1x16 Ohm, 1x8 Ohm, 1x4 Ohm, 2x16 Ohm or
2x8 Ohm. Any other speaker configuration may
stress the power amplifier section and in
extreme cases may lead to valve and/or output
transformer failure.
HINTS & TIPS
Factory Reset
This will erase all the MIDI presets and set MIDI