Valve amplifiers are usually load sensitive because the load connected across the output is directly across a transformer which is also the load for the output valves. The output valves are designed to work into a particular impedance which is provided by the transformer’s impedance ratio multiplying the load impedance. Thus if you change the load impedance you change the working conditions for the output valves.
The valves can work safely within certain limits. The harder they are working, the more vulnerable valves become. If the load is too low an impedance the valves are closer to a short circuit and pass too much current. If the impedance is too high the valves are usually safe, as they draw less current and hence are not working hard, but an open circuit will kill them dead and a load that is too high can cause over-volting of the valves and transformer.
This depends on how hard the valves are being driven, but while an 8R output may be ok with a 16R load, a 4R output into a 30R load may not be a good idea!
In general, the load, usually a speaker or load box, should match the amplifier output. It is also preferable for quality and reliability that the load should be of a similar nature to a speaker when using valve guitar amplifiers. A plain resistive load can result in destructive effects within the amplifier output stage. A suitable reactive load is best, and can be appreciated from the natural sound produced.
The load as seen by the amplifier will be the total effective impedance across the output connectors. This is determined by adding together loads connected in series, like a chain, or dividing loads connected in parallel (load impedance divided by number of cabs), provided all the cabs are the same impedance. Mixing cabs of different impedance is a bit unpredictable as they will not share power properly.
The Motherload Elemental is available as 4, 8, 16R versions as standard. 2R can be made to order.