I'm about to make a box for my IDQ10 d2 v2 that I'm going to run off a 400WRMS amp and fire it through the armrest hole directly into the cabin of a small sedan.
I.D. rate nominal power handling at 250WRMS and recommend a sealed box 15-26L.
Should I make it smaller for power handling or a bit larger so it can go a bit lower? The best overall S.Q.is my goal.

The IDQ subs actually like ported box's. They will sound just as tight in a ported box. Cept ported will go deeper, louder and make the sub more sensitive.

I would go with 28litres tuned low at about 30hz. It will go very very deep with that setup

Mine is in a 0.5cuft box and sounds wonderful, these subs just love small boxs.

The sub has a very low Qts this is why they like tiny sealed box's.

If a sub is designed to work in a ported box and sounds good ported then definitely port it. The benifits of porting a box far outweigh the deficits. Thats for both Sound quality and SPL

Maz If I did decide to port the box,with the sub behind the back seat firing forward through the armrest hole would I port it through the parcel shelf?
Is it really a good idea to exceed the rated power of a sub in a ported box without a subsonic filter?

A subsonic filter is only needed when u play music that contains sound below the tuning frequency. I highly doubt there will be much if any music below 30hz.

If there was it would be very little.

Also the sub moves LESS with a ported box buts actually goes LOUDER. So a ported box can take even more power before the sub reaches max excursion

When porting u could go tgrough the parcel shelf, that would be easy i guess with some 4inch PVC pipe. The port doens't have to be entirely in the box so it can work sweet.