Hi!
I don't know about other two songs but I know A-ha's "take On Me" and the bassline is played on old Yamaha's DX/TX FM synthesizers. By the sound of it it is definitely a classic 12-bit, 6-operator FM tone generation.
However, any FM synth should be able to do this sound without a problem.
The one I would recommend the most is Native Instruments FM7. It can import all the original Yamaha DX/TX sounds and it is probably the best FM synth out there.
There are other ways of getting this sound as it is most likely featured in any synth bass sample library out there (it is a classic) but FM7 will do a better job. FM is best done with FM.
Also, keep in mind that most of the stuff in the 80s was heavily overdubbed and processed with FX. The bass in A-ha's song is probably overdubbed, chorused, EQed andd I'm willing to bet that it is actually one or two (if not more) DX synths playing the same part.
-ED-
P.S.
Just checked out the other two songs. Typical 80s DX slap bass. So 80s!!!
[This message has been edited by 3351 (edited 02-16-2006).]