You'll have to take it in to get it repaired. From what you described it sounds like you blew the output stage out. You can't run from your speaker output jack into the input jack of another amp. You should go out of the recording out, line out, effects out or in the least case your headphone out. What happens usually is your output transformer short circuits due to a mismatch of impedances. It sounds like you still have a transformer though so you might have gotten lucky. You will need to take it in though. Sorry.

Are the external speakers of the same ohms and wattages? Has it occured to you maybe the orignal speaker is open? Check the ohms that are marked on the speaker with a DMM set at 200 ohms. Power off amp of course. And check the electrolytic cap that is connected to the speaker, These are safety caps and are used to protect the speaker. If you don't have a cap checker...replace it with one with the same Mf and voltage and use a 105 degree cap which is better than an 85 degree cap. This is a cheap check and could be the problem. Good Luck!