I did a little reading and found a nice article in Sound on Sound about impedances. Below is a link and quoted is the section specific to valve amplifiers though not specific to Bottlehead designs. I placed a 4 Ohm resistor load across each channel and notice small changes. if the amp is happier like this then I'll leave it in place.
https://www.soundonsound.com/techniques/understanding-impedance "Whereas most modern solid-state amplifiers are virtually bombproof in terms of whether their outputs see proper loudspeakers (of any nominal impedance) or a short or open circuit, most valve amplifiers are far less tolerant. In fact, the majority of vintage valve amps will self-destruct if driven without the correct speaker load attached! The reasons are complex and depend to some extent on the design of the output circuit, but can be boiled down to what are called 'reflected' impedances.
Most, if not all, practical valve amplifiers employ an output transformer. The use of the transformer is principally to translate the effective load impedance between that required by the valves, and that of a practical loudspeaker — a typical valve output circuit requires a load of between 5kΩ and 10kΩ, whereas a practical loudspeaker presents a nominal impedance of between 4Ω and 15Ω. The transformer does this by 'reflecting' the loudspeaker's impedance through the transformer (as a function of the square of its turns ratio) to create a different (in this application, higher) load impedance for the valve output stage. Thus a 15Ω speaker will appear to the output valves as a 9kΩ load, say. It is important to note that it is the physical loudspeaker's impedance that defines the operating load for the output stage, and that valve amplifiers are very fussy about their load impedance. If a loudspeaker with a different speaker is connected, the output valves will see a different load and their performance and operating characteristics will change as a result.
Consequently, to make the system more flexible in accommodating different loudspeakers, many valve amplifiers have different output terminals (or some way of selecting nominal output impedances) for different loudspeaker loads. This is achieved by using different tappings on the output transformer so that an 8Ω speaker connected to the correct terminals will produce the same reflected impedance to the output stage as a 15Ω speaker connected to its appropriate terminals.
So what happens if the loudspeaker is disconnected? Well, instead of the 15Ω load being reflected into a 9kΩ load for the valves, we now have an infinite load, which will be reflected as an infinite load to the valves. For a given current, an infinite load requires an infinite voltage. Imagine a brief positive transient audio signal (a drum strike, perhaps) driving the output valves to the unloaded output transformer. When that transient stops, the magnetic field developed in the transformer collapses and generates a reverse polarity signal called the 'back EMF'. With an infinite load impedance, the back EMF will tend towards an infinite reversed voltage spike and this is applied directly to the valve anode plate. Depending on the valve in use, this huge back EMF is likely to far exceed its rated values and so may cause the valve to break down, damaging or destroying the grids or anode plate, and resulting in one very poorly amplifier.
However, this huge back EMF can only be generated if the amplifier is being driven in the first place. If there is no input signal to the amplifier, there will be no output signal, and so no back EMF. Under these rather exceptional circumstances there is unlikely to be any damage. The most sensible thing to do, however, is to always check that a suitable loudspeaker is connected to a valve amplifier (with the correct output terminals or transformer taps selected), before you connect or turn up the input."
