The extra speaker jack can be easily converted to a line out. Gerald Weber's tube amp book tells you how to do this in the chapter on Tweed Deluxes. Here's how to do it:
Disconnect the wire(s) connecting the ext. spkr jack to the main one.
Connect the tip connection terminals of both jacks using a 2.2K 1/2W resistor.
Connect the tip and ground connection tabs of the ext. spkr jack using a 100 ohm 1/2W resistor. You're done.

You still need to keep a speaker plugged into the main spkr jack in order to maintain the proper load on the output transformer/power tubes.
Also, best results will be obtained if you first route the line out signal from the amp to a speaker/mic simulator box (such as a Hughes & Kettner Red Box) before sending it to the mixing board. Such a box will do two things:
(1) Filter the signal in a way that mimics the coloration you get from micing a speaker (otherwise the tone will be too bright and harsh); and 
(2) provide a balanced, low-impedance signal (XLR connector) that you can send to the board using a standard mic cable.

Just choose an appropriate series resistance, depending on whether you want a line level signal or a guitar level signal (to jumper into another amp), in conjunction with a low resistance pot. 

I've had good luck with a 47k series resistor to a miniature 1k pot to ground. Output from wiper is adjustable for many applications. I think a Boogie MK3 had an adjustable line out with a pot, but don't recall the values used there. 
I posted a description of this device here about a year ago. The most versatile one (portable) can be built in a tiny aluminum minibox, with 1/4" jacks for input & output. Inside the box is a terminal strip to mount the fixed resistor. The pot is mounted so the knob is on the top, jacks are at each end. The input cable can be made up using alligator clips on one end for amps w/o ext speaker jacks like Champs, to clip right on the speaker lugs.
Output cable can be any guitar cable, or some special rig with phono
plugs to go into a cassette recorder, etc.