"We have our fair share of hoards in Norfolk, but this is so, so different - it is really, really special," he added.
Dig leader Peter Crawley said he had a gut feeling about the site before the excavation began.
"It felt like one of those special sites that you do get every so often," he said.
A war trumpet was used by Celtic tribes across Europe to intimidate their enemies and to inspire and direct warriors during battle.
They included the Iceni tribe of East Anglia, which was led by queen Boudica in her failed uprising against the Romans in AD60 to 61.
Carnyces fascinated the Romans, who frequently depicted them as war trophies.
Conservator Jonathan Carr said the Thetford find was made "from extremely thin sheets of metal which have become extremely brittle after 2,000 years in the ground".
As well as being one of only three carnyces found in Britain, it was "the most complete carnyx ever found, with the pipe, mouthpiece and bell all uniquely intact", he added.
Dr Fraser Hunter, Iron Age and Roman curator at National Museums Scotland, said: "I've looked at carnyces from around Europe, and the full research and conservation of these incredibly fragile remains will reshape our view of sound and music in the Iron Age."
The boar's head standard was made from sheet bronze. It would have been held aloft in battle like a flag and used as a rallying point for troops.
Hunter said: "They were very fierce animals to face in the hunt, so the symbolism of the boar is a lot about the strength of it - a very appropriate adversary in battle."
The hoard, which dates to about 50BC and AD50, included five shield bosses and an iron object of unknown origin.
Following its discovery, it was carefully lifted out of the earth in a soil block and scanned to work out where the items were positioned, before conservation began.
The find was reported to the coroner and an inquest will be held to determine if it is treasure. Its long-term future has yet to be decided.
Historic England is working with Pre-Construct Archaeology, Norfolk Museums Service and the National Museum of Scotland to coordinate the finds' research and conservation.