That would make sense.
Since Brexit, there is a Free Trade Agreement in place between the EU and the UK, which means no import duty is levied on goods traded between them. However, that exemption applies only to goods that are made either in the EU or in the UK. Goods that are from other parts of the world, such as China, get duty levied on them accordingly.
That being said, it appears that the third-country tariff duty (China is a 'third-country' in this context) on 'audio-frequency electric amplifiers' which appears to be the best match for a guitar amp [Commodity code 8518400090] is set at 0%, so no import duty is due, whether your amplifier is originally British, German, Chinese or actually hails from Betelgeuse.
https://www.trade-tariff.service.gov.uk/commodities/8518400090
FWIW, from what that I have seen, most import tariffs on most products are below 5% most of the time.
Finally, don't get duty confused with VAT. Regardless of any duty, rules of origin or anything else, whoever is responsible for the goods entering the UK will have to pay VAT upon import. That is almost certainly going to be at 20% Standard Rate and will be a much bigger hit to your pocket than any duty. I've spent much of the last year rewriting my company's trading systems to accommodate Brexit and VAT is a much bigger headache to deal with than duty.