accompanied by a rather nasty smell of overheated electronic components! Switched back on it gives the usual slight hum, but the power light doesn't come on, and it makes no sound. Where can it be repaired, or is it dead? I live in Dunedin. Thanks for any advice.
Yes its dead
Its a garbage design (or garbage components) if the amp blew that easily . (Ive build a few amps back many years ago)
You'll need someone experienced in amp repairs , not some local handyman or auto electrician
A guitar amp repairer or PA amp repairer would have the skills to repair it.
If its a Class-D amp , may be not be repairable (easily)
It may not be worth the cost of repairs . Wharfedale subs are often pretty cheap to buy on trademe