The parallel out should have sounded the same as the other jack on the 212 speaker cab. If you were having issues, was the speaker cable plugged into the proper output jack on your amp? Was it a speaker cable or instrument cable? I generally use a heavy speaker cable with the extra large plug housings so they do not get mixed up with any of the instrument cables. Never use an instrument cable to connect speakers to an amp. Instrument cable uses 50ohm coax where as a dedicated speaker cable is a pair of heavy gauge wire (18AWG - 12AWG depending on brand) with extremely low resistance.
When ever you remove speaker cable, always double check where it plugs into the amp so you do not end up with an open output transformer. It is too easy to accidently plug into the return jack by mistake as it is right next to the 8 ohm output jack.
412 users only:
I would only indicate one word of caution when it comes to the 412 cabinet. The parallel out may be open circuit if there is no cable plugged into the mono input as this one jack has a built in switch to connect the left and right side speakers in series. Also do not use the parallel out of a 412 cab when using the stereo inputs.
I still have the small caps in place on the unused jacks as it is harder to install the speaker cable in a plugged hole if it is too dark to see what you are doing when you set up.