u99->6176 is my normal vocal chain... sounds great for rock stuff. Have to be very careful not to distort the pre, but it sounds amazing...just have to pull a little low end off and the vox seem to sit pretty well as the mic is pretty bright. i love to spank it though the 6176 again through mixing especially for the crushing distortion the tubes can generate.
I use a U99b along with a M610, and depending on the singer/singing style it can be a really good sounding combo. on softer singing styles however I feel the need to roll off some bass to make the U99b not overly bassy. maybe something with a more forward upper midrange like a daking would be the perfect choice. I do not have the money right now to get a bright preamp like a daking, so I live with eq....not bad anyway! the results kick ass, the U99b has a totally in your face sound, fullsize, love it! and I have to agree, the M610 really kills as a bass preamp!