The 140 has two parallel speaker outputs (1 and 2) and a "booster" output, which is the same signal as the speakers get but attenuated down to a level where it can be used to drive an external power amp. It's similar to a line out.
That amp was intended to be paired with the Acoustic 106 cabinet, which was a 2x15 cab. Those old Acoustic cabinets are very good-sounding cabinets. Later Acoustic also made a similar cabinet called the model 402. A model 404 1x15 cabinet would also work well with that amp. Of course the bigger model 406 cabinet (2x15 in a bigger box) would also be good, although tougher to move around. You couldn't go wrong with any of those (106, 402, 404, 406), really. I don't have personal experience with the new Acoustic cabinets that they sell now at Guitar Center (B115, B410, B810), although they tend to get good reviews and would match your head in general appearance. The B115 and B410 are 8 ohms, and the older cabinets are 4 ohms, so if you're running just one cabinet, you'll get more power out of the head into one of the old cabinets, unless you go with the B810. Although, it's surprisingly loud for its very conservative power rating, even into 8 ohms. You could always get two of the new cabinets to run it into a 4 ohm load. :-)
For more information, check out the Acoustic Wiki at http://acoustic.homeunix.net/twiki/b...oustic/WebHome
There's lots and lots of information about Acoustic gear there, particularly the "original" series of Acoustic amps and cabinets.