When it comes to brands and specific products, I think it's more consistent with other Danbooru tags if we instead use brand names as tags, not brand_amplifier.
For example, we have Samsung instead of samsung_cellphone or samsung_television, and we have Volvo instead of volvo_car or volvo_truck. We also have specific model tags like Samsung Galaxy S24 and Volvo FH so if we wish to have specific amp model tags we can, like how Fender has guitar model tags.
For brands like Marshall and Fender, you'll find them on many types of devices, and the advantage of having brand name tags is that you can combine marshall tag with amplifier or speaker, which is tidy when tagging and searching.
As of currently, tag Orange Amplifiers refers to the brand Orange Amps, but I'm not sure if the company has officially clarified whether "amps" stands for "amplifiers", or "amplifications" as used here. I'm inclined to believe "Orange Amps" is the full official name.
Marshall Amplification is recognized as the full name of the company though, see Twitter and Instagram. I think it's fine to keep using this name even for products that are not amplifiers.
Following Fender's example we can consider making the Orange and Marshall tags copyrights.
I'm going to create a BUR below accordingly and we can vote to decide the preferred practice.