The K and E series are newer versions of the D series and either would be a good alternative. It would be most important to find a D140 if you were restoring an older Sunn cabinet to original specs, otherwise, any 140 model should do fine. If you buy a used JBL speaker, make sure it has a JBL cone in it. I own a number of JBL 15's and have learned the hard way that what I thought was a great deal turned out to be a recone with non JBL parts. Be aware of non JBL recones, their sound and power handling capabilities are far inferior. I have a K140 that I've used for 20 years, and it still sounds great.