Peavey Vypyr external Cab

I have a 40W SS vypyr/112.  internet rumor is that I can get more volume w/ an external cab.

can you recommend a pair of 12" speakers for this project [minimum weight maximum volume]? 

to achieve 8 ohms, is it better to have 2 16 ohm or 2 4 ohm [i.e parallel or series]?  thanks for any suggestions or help.

Hummm ... well ... "minimum weight maximum volume" would be a pair of G12 ... but they wouldn't sound great with that amp!   So, I'm going to suggest a pair of Retro 30s!