2x12, MUST BE: Transparent, featherweight, 4ohm, vertically aligned.
First of all, I would not use the word "transparent" as the cabinet colors the sound a lot. One and probably the most important reason for this is efficiency. Think about a hifi system, that needs to have as straight freq response as possible. They may have the efficiency in the ballpark of 0.01 % i.e. 100 amplifier watts equal 0.01 W of speaker noise and the rest (99.99 W !) will warm your house through speakers' voice coils. This happens, as the response has to be tweaked with components in the speaker x-over.
Now, a PA system (including bass and guitar systems) may have efficiency in the area of 1 - 2 % (and 98 - 99 watts warm up the club house). This is some 100 times more sound/volume/noise coming out from the system compared to hifi. Is it fair to compare f response of these two? Certainly not. So please, do leave the word "transparent" out.
If you are into technology at all, you probably already know that impedance does not actually matter volume-wise, as the volume difference between those two (4/8 ohms) is mostly negligible. In this real life a 4 ohm cabinet may produce LESS sound that its 8 ohm counterpart at high power levels but this is amp dependent: sometimes 4 ohms is just too hard for the power amp. So as a specification, is it a reasonable must? That's why I would write: 2x12, featherweight, vertically aligned, (adjustable tweeter optional).
One speaker suggestion to the list would be Italian Alusonic - and was Swedish TKS mentioned here?
http://www.alusonic.com/
http://tks.se/