There was a company that made tube retrofits for the U87 a few years ago.. I can't remember the name however..
There are lots of places that have the older vintage stuff, Vintage King, Blevens Audio, Wes Dooley perhaps. The best source for all of them and that kind of thing is http://www.proaudiomarketplace.com

It costs $25 to subscribe but it's well worth it if your looking for used and vintage gear. One of the best sources there is..

[ November 29, 2003, 02:18 PM: Message edited by: Kurt Foster ]

Soundelux makes the U99, which I've heard is in the ballpark of the U67, but it ain't cheap.

The company that makes the tube amplifier for u-87s in Innertube auido. I do some work in a studio that has one and a soundelux U-99. I like the Soundelux better but it cuold be that the U87 needs work