I know this is a well covered topic, so I'll keep my question precise.

I have three Z9 Minis and want to quieten them down as much as possible (I understand the effect will be limited).
I will build a single plywood box lined with a double layer of soundproof plasterboard.
I will make cut large rectangle vents in the ends of the box and form angled ducting using the same construction method (ply box section with double gypsum lining). The angle is an attempt to deaden the noise from the vent holes.

Q) How much improvement can I expect to achieve with an additional internal layer of mass loaded vinyl in the box and rigid ducting?

Thanks for any qualitative (or even quantitative) responses.