Don't think the AC plug is the problem. C1A should definitely greatly peak a weak station at the high end of the band. Right now NostalgiaAir is down but schematic can be seen at
http://techpreservation.dyndns.org/s...53-561_80.djvu
Check that the loop is connected properly. Per the schematic, the outside winding of the loop goes through a 944 mmfd. (or pf) cap shown as C2 on the pictorial to the stator of the rear section of the tuning cap. Also through the bandswitch when in broadcast band position the outside winding gets a direct connection to the rear stator. The inside winding of the loop connects directly to the tuning cap rotor (frame.) On the pictorial attached to the schematic I don't see that wire shown, and wonder if the inside of the loop got inadvertently connected to the outer end of L2? It is connected there, but also perhaps through internal wiring to the cap frame? Hard to tell without tracing the underchassis wiring.
What were the two rectangular caps you replaced, and what values? The old ones may be OK.