Yes, you can run any aftermarket speakers without any other modification necessary. I'm not sure what you mean by "replacement amp," though... if you mean aftermarket or external, the answer is still yes in both cases.
Tyler R. - Ansonia, CT
Silver '12 JKRU
6-speed, soft top, 2.5" TF Lift, 35" Treadwright Guard Dogs, Smittybilt SRC front bumper, Superwinch TS9500 winch, M.O.R.E. oil pan/trans skid, Skid Row exhaust loop skid, Dominion evap relocation,*Smittybilt Oversized Tiire Carrier and Cibie headlights.