It depends on the frequency--I think you get better reach the further left you are on the dial. Also the height of an antenna varies with the frequency. If you're on I-89 in Waterbury VT you can see the fairly tall towers of WDEV 550 while a station at, say, 1490 might not need such a tall tower.
I know WNSH 1570 Beverly is 85 w at night and barely covers its city of lic. (30kW day)
You asked about 14w night. Well WJIB is 5w night and here is the coverage map:
http://radio-locator.com/cgi-bin/pat?call=WJIB&service=AM&status=L&hours=N
From Brookline to Medford primary, and from around Needham to just N of Woburn fringe. That's 5w
not 14 and it's on 740. I'm not sure they'd reach as far with those 5 watts if they were further down
on the dial
WJIB has 250w day. You asked for 500. I can't think of any 500s offhand right now but WJIB 740 day is
http://radio-locator.com/cgi-bin/pat?call=WJIB&service=AM&status=L&hours=D
The diagram has a measuring unit showing 20 miles to give you an idea
If WKND moved from 1230 to 1480 I'd guess their coverage would be smaller
Here is WKND's 14w/night coverage chart
http://radio-locator.com/cgi-bin/pat?call=WKND&service=AM&status=L&hours=N
Days
http://radio-locator.com/cgi-bin/pat?call=WKND&service=AM&status=L&hours=D