A lot of people think because an amp has a high wattage rating that you can't use it in a smaller situation and before modern, now technically ancient designs, that had some merit, depending on where in the power curve of the amp the end-user chose to get "his/her" sound. Each stage of an amp is part of a system chain that colors the output of the guitar pickup each based on its gain, power source stability, and filtering. The result you hear is a summation of the chain with a final filter, the speaker, cabinet, and where you place your ear relative to the sound wave. Hiwatt, like big bottle audiophile Marantz amps, did not saturate the pickup output in the earlier stages of the chain, so you get a high fidelity signal all the way through. Hiwatt used inefficient Fane speakers which when driven hard would break up and color the sound wave to give the characteristic sound. Inefficient does not mean beshissen, but the power delivered to the speaker is not accurately seen in the output of the sound wave. There is a unique coloration.
A big bottle amp, referring to the output tubes, sounds much different when run is a triode mode (less wattage) than pentode mode. Take any amp with a power selection and compare what you hear at the different wattage settings. You will likely prefer the full power setting because the others do not deliver the full fidelity of the initial part of the signal chain. You hear something not quite as warm, with a part of the bloom missing. The point is greater power leads to higher fidelity even when it is not cranked.
I came close to taking a plunge in the Hiwatt pool, but didn't want to fight a front end designed for clarity, one which lacked touch sensitivity. Listen to the guy's clips of mainly the guitar without effects and you will hear it die early, no sustain, which is part guitar part feedback within the amp. My experience, yours may be different.