There was
a Groove Tubes ViPre that just sold for $1300 on eBay earlier today. I was soooooo tempted to get it, but I already have enough decent preamps, including the Groove Tubes SuPre, which isn't quite as top-end as the ViPre, but it is a very nice tube preamp with a very Hi-Fi vibe.
Anyway...the point I want to make is, you might consider some of the preamps (like the GT stuff) that have variable impedances and/or some sort of on-board EQ...that way you will get the most out of that U87.
Also...before you buy the U87, I will suggest you check out the ADK TT/CE, which is a multi-pattern tube mic that kills, costs much less than the U87, and according to some folks...sounds better:
U87 vs ADK CE small listening test - Gearslutz.com
I have the CE (along with a bunch of other ADK mics) and it's my #1 vocal mic. After owning it for about 7 years, last summer I decided to put one of the ADK Custom Shop capsules into it, and it took it to an even higher level of tonal sweetness.
The cool thing was that I bought another ADK mic on eBay for almost nothing, because it had a cracked capsule, and I dropped my original CE capsule into it...so it worked out perfectly.
Another tube-ish brand of preamp that you might consider that gets good reviews, but is not a real major name-brand, is the Sebatron stuff. Made in Australia, they do a tube front end with a solid state output...very nice. I have a 4-channel model, and I've used it on just about everything.
My favorite combination so far is my ADK CE through the Sebatron...it's just so rich and smooth sounding.