I don't know about either of those malts specifically although at 27L I don't know how much diastatic power the amber malt will have left. The mild malt should convert just fine, maybe take a bit longer.
I made a 100% munich english barley wine once and it was okay. super malty. a bit too sweet but that might not have been entirely the fault of the munich.
I'd go for it with the mild malt. There is an old Barclay Perkins recipe from the late 1800's that uses 100% mild malt for a strong (XXX) ale.