I own an older model JCA 20 watt combo (controls on top), for which I ordered the matching extension cabinet for.

When it came, it had a small board you can attach to the back to make it ported. This was evidently done by THQ.

Long story short: closed back, open back (bottom half of back gone) or ported (smaller horizontal opening in the lower half of the cab)? I play program rock, maybe some heavier stuff. If I'm looking to get as much volume from the 20 watts, I should port it, right?
Found a picture of what it looks like when ported, closed is a full back panel and open is removing the bottom piece.
^ TS was asking if he should port it, not if it is or isn't ported.

Try everything TS, and keep what sounds best to you, 'cause it's really mostly a matter of preference.

Ideally the more close it is the tighter the sound will be.
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^ Yeah. If you can be bothered, try them all. if you can't, I'd suspect closed for heavier (though smaller cabs like 1x12s can sometimes sound boxy closed, so you basically have to play it by ear to a certain extent).
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I run a JCA20H through a closed back 112 with a Celestion Vintage 30. Sounds great to me, but your cab could be a whole different world.
Vintage V-100, EMG 81&60
Chapman ML-1

Jet City JCA20H
Closed it was tighter, open it was typical sounding, and ported it had the rumble of the gods, if just a little boomy.

Thanks guys!