Can I connect a DC power supply to Line 6 Micro Spider with AC "port"?

Your amp only has one power supply port, designed to take the OUTPUT* plug of a wall wart AC/DC converter (ie: the PX-2g).

jea48 is correct! The PX-2G is a transformer step down 120VAC to 9VAC, it's not a wall wart AC/DC converter.

The, "AC IN" labeling is an error and the company should made aware of THEIR confusion (AS IF they care, over there).

                                         *AC in / DC out, of course.

No, the "AC IN" labeling is not an error! The Line 6 Micro Spider amp will convert the 9VAC to DC internally. 

BTW, a wall wart AC to DC converter may work as well, but after the Line 6 Micro Spider amp's internal rectifier, a 9VDC input will output much lower DC voltage than a 9VAC input. You need 12.7VDC input to get the same DC voltage after the rectifier of a 9VAC input.

       Having read the output spec on their transformer incorrectly, never having seen a 2.1 mm power jack used for an AC input and that the amp will also run on  six C-Cells: I made an erroneous assumption.  (DUH/Old-Timers' Disease)

                                     Have to give Line 6 credit though.   

        For the cost of one rectifier per unit, they've probably compelled (most) anyone purchasing any of the eleven products in their 9V line and not wanting to buy stock in a battery company, to pay an additional $39.99 for the PX-2G.

                                                           SLICK!