Howdy Otterfunk! First, I think you got a great deal on your 200S rig, well done and good luck! I think you will find the old tube Sunn equipment is about as good as it gets from a bass tone perspective. I have just gone through a similar experience with a 2000S cabinet, I bought it loaded with an Eminence and CTS 15" drivers. Actually it didn't sound awful as it was, good cabinet design. Anyway, I think the K140 is an excellent choice as a replacement. The frame and motor assembly is identical spec wise to the D140, same alnico magnet, but the surround is less resilient and the cone is a tad stiffer, making the speaker a little more percussive sounding, not quite as "warm" as the D140. I personally like it as it has a little tighter sound, more modern sounding but still retaining the smooth alnico attack characteristics. The best way I've found to get a good speaker is to get it from a reliable source, someone who stands behind their products. Keep in mind you are dealing with old stuff, and there is always going to be a little gambling involved! Here's a source;
http://www.jamminjersey.com/speakers.php?prod=jblspeakerI've had good luck with them, and they really know their stuff. I use 2 16ohm K140's wired in parallel to get 8ohms with my 120 watt 2000S head, but I use 2 8ohm D140's in parallel to give a 4ohm load to my 60 watt 200S. It depends on how hard you want the output tubes to work, I use the 2000S for a solid clean deep bass tone and use my 200S to get a little more overdriven tone.
I hope that this helps you out. Feel free to email me if you have any other questions. I’m no expert, but I’m happy to assist you if I can!
By the way, it's probably a mute point by now, but the K140 in the ebay auction looked stock and healthy except what appeared to be a white sealing strip on the inside of the frame rim. They probably had it mounted in a front loading cabinet.
