I will say the Supersprint racing backbox is flipping loud, hence why it sounded best with everything else in place. Nice deep sound, not raspy, few pops and minimal drone from what I remember. So the more 'sedate' backboxes out there might not make enough of a difference with all the other bits in place. Though your decat will help a bit.
I got greedy though and wanted more pops, and from that point is when things spiralled! So I fitted the matching Supersprint racing centre (non-resonated), sounded great but too loud and man it droned! I then fitted an R3 manifold (so pre-cat delete), and changed the stock cat to a sports cat. It was like flipping armageddon!! The only silencing was a sports cat (which does f'all) and the racing backbox (also did f'all!). BUT, it sounded awful, so raspy and farty, like a riced out 90's Honda
So I then added a centre box and changed the sports cat for a decat (which is what I run now). It's loud (too loud still) and drones a lot, but the noise is sweet, and the pops many! BUT,it doesn't perform very well on the dyno at all!!
The point of this tale....I've now spent enough to have bought an Akra, and yet I have something clearly worse in sound and performance. So you could take a punt on changing the backbox only, the custom route will be very unknown on the sound too, then maybe you'll make some other changes to get it how you want, perhaps get lucky or like me make it worse! So if you're after a good sound that's not too loud.... just buy the Akra and be done with it?!