You can get some very nice low wattage tube amps that in my opinion would be an improvement over your HT-5, and that's the kind of thing us here on the forum would recommend e.g.
Orange Tiny Terror
Hayden Mofo
MJW Roadstar (he's a UK custom amp builder)
A vintage Fender Deluxe
Mesa Transatlantic
Cornell Plexi 7
etc
etc
etc
Always remember too, that a £200 pickup swap won't turn a £400 guitar into a £2000 one. I always refer to pickups as the "salt and pepper" on a meal. It won't turn a salad into a steak, but it would make a steak taste a bit nicer ;)