I've changed a fair bit:
*Autodelta drop-in air filter: negligible difference in noise; good that it's cleanable and reusable
*backbox: big difference, but at risk of excessive drone; I have a local Oz-produced HiTech Muffler 'Mille Miglia' backbox
*centre pipe: I use the factory original with the resonator; some people go straight-through, which makes the sound louder, but little change in character; if you simply want 'more of the same', then you could keep the factory backbox, and change to a straight-through centre pipe, and it will be a bit louder
*highflow main catalytic converters: I have 200 cell/in^2 HiTech Muffler highflow cats; this helped reduce drone and 'refine' the sound
*equal-length front pipes (between manifolds and cats): added high rev growl and a bit of metallic snarl; I liked the effect of these, plus they provided (easily) the biggest power increase that I could feel (haven't dyno'd the car since adding these, but I dyno'd after the backbox and cats and the power increase was very modest and not obvious from 'the seat of my pants')
I have no plans to change the manifolds. I've read mixed reports; whilst it increases power/torque, it can ruin the sound.
So I'm really happy. The backbox alone made the biggest difference in sound, but there was (and still is) drone at about 2500rpm. The cats and front pipes reduced this a lot and it now sounds great.
This was before the equal-length FPs: YouTube - Alfa GT 3.2L Mille Miglia high flow cats + rear muffler (1)
When you hear it in person, it's quite loud and can be menacing, but all in good ways!