With the advent of cheap glass D amp technology, any modern box SHOULD blow ANY vintage box into the weeds for sheer loudness and bass production but the GPO box would be thrashed but many vintage heavy hitters.
But any marketing guru will tell you the look, number of features and the amount of shiny bling will always win over sound quality and RMS output numbers.
The GPO would make an ideal amp and power supply (battery) upgrade project. You’d have to toss the factory speakers but imagine that thing with a pair of fat, chrome capped drivers?! Carve out the tweeter hole and mount some nice silk dome tweeters and some good crossovers and you’d be all set for any street battle.
I love the vintage look, it just needs more ponies under the hood.