Hmmm... General consensus is the stock speaker is quite good until you push it (I agree). You have to be careful selecting a speaker for an amp with a modelling front end as the models will be voiced specifically for the speaker and an "upgrade" isn't always that because of it. The usual recommendation as an upgrade is the Eminence Ragin Cajun (available on Take2 for under R1400). Be aware that it will make the amp a fair bit louder though.
Also what impedance is the Champ? I see the Greenback is a 16 Ohm. BTW - Ryan's selling an
old pair of Greenbacks for R1500 in the classifieds.