Just to get the clear legalities of the situation out there for all:
If you do not declare goods imported into the United States they can be summarily seized and destroyed.
An amplifier bought from the UK could be charged 4.9% (
Harmonized Tariff Schedule of the US) duty on it's declared, or assessed value. Customs can assess the value on their own.
Since this is an SR71 you could go this route.
If an article is made in the United States, subsequently exported, and then returned to the US, without having been "advanced in value or improved in condition" by any process of manufacture or other means while out of the country, it is not subject to duty upon its return to the United States (Harmonized Tariff Classification 9801.00.10). NOTE: this may delay your item as CBP may require proof of no-advancement in value from the orginal manufacturer.