Having built mmy first 2x12 guitar cab, the sound isnt what I expected it to be.
I used 18mm plywood with battens screwed and glued inside for a strong job. Speakers chosen were Celestion Vintage 30 and Celestion G12-H. Wiring from jack to speakers done correct also.
Sizes of cab are550mm high, 765mm wide and 330mm deep.
I own a 4x12 which I played for years, loaded with Celestion Vintage 30s. My Marshall treble setting was always 12 o clock, but with my new 2x12 the treble has to be full. Do new speakers need "playing in"?
The overall sound is a little "boxy" too. Are all 2x12 cabs the same as regards boxiness?

Course it's not the same! The internal volume, type of cab (as in sealed, ported or openback) all have a huge impact on what it will sound like. Each driver is designed to operate in specific circumstances. Mixing two different drivers is also a bit unusual. There are computer simulators that will help predict results, but guitar cabs are quite specific. You can often take an existing design, then alter the dimensions, but keeping the internal volume the same and it sounds similar - but never the same. The density of the cab also has an impact, as do bracing and wadding. Port tuning in semi-sealed cabs has an impact. Open back guitar cabs usually have a thinner sound than a 4 x 12 cab. If you look at processors like Line6 and Behringer, you'll see speaker cab simulation is high on the variable list.

How did you choose the sizes you used?

Haha, that really covered everything I was going to say.

Look in your local book shops, or amazon, you can find loads of books about speaker cabinet design. it is a tricky art.

Also speakers work best with a certain amount of space in the cab, different for different speakers, you'll need to look in to that.

But honestly, buy the books, read up on it.

Or ideally ask someone who knows about cab building already.

I have a Marshall 2 x 12 and 2 x Marshall 4 x 12s. They all sound very different even though they all have the "same" drivers. The 2 x 12 is the least nice sounding imo, lack of bass and it is brighter although not as bright as the angled 4 x 12.

Newer Celestions don't sound as good as the old ones which is why genuine vintage ('60s) Celestions go for really stupid money on eBay.

I'd reckon they will "play in" but it will take some time depending on how much you use them.