confused on ohms

[rightintheface]

so i've decided to go with the avatar 2x12 cab (not sure what cosmetic details yet, perhaps burgundy and oxblood with silver piping) over the mesa vs orange thread i started the other day. this will be my first cab purchase, i've been playing combos for a few years, so don't really know what would be "best" to go for in terms of what ohm rating i choose for the cab.
i'm running a 50w Hughes and Kettner switchblade combo. the plan was to hook the 2x12 into one of the speaker outputs, and still run the combo's speaker.
the outputs are: 1x 4 Ohms, 1x 8 Ohms or 2x 16 Ohms, and 1x 16 Ohms. the combo speaker (eminence rockdriver 60) is connected to one of the outputs labelled under "1x 8 Ohms or 2x 16 Ohms", and i'm not sure what rating it is. the standard avatar cabs are 8ohm, but can be ordered in 4 or 16 ohms.
to be honest, i have absolutely no idea which i should get  :-\  i am a complete noob when it comes to cabs/ohms etc etc
i'm also keen to add another 2x12 further down the line. the cabs can be daisychained and attached to half the ohm output ie. 2x 8 ohm cabs daisychained connect to a 4ohm output.

any suggestions or comments?  :dontknow:

 

[jackthehack]

As the Avatar cab is rated at 8 ohms, you'd plug into the 1x8ohms jack

 

[rightintheface]

i'd worked that much out, but, would it still allow me to use the combo's internal speaker? it's attatched at the back with a normal 1/4" jack, so it can be unplugged if needed, but i'd like to run all three speakers at once.
are there any (dis)/advantages to the different ohm ratings?

 

[jackthehack]

Which connector is the speaker plugged into? I'm unfamiliar with this brand of amp. consult users manual about external cabs. You should use appropriate ext. jack to put proper impedance load on output transformer to avoid issues and maintain speaker efficiency.