If you let us know what the rest of your signal chain is it might help in getting better advice.
I don't know anything about the Grace unit but the Presonus one I have used, it added a smal amount of "warmth" but was not an outstanding difference IMO. This DP unit is a stereo (dual input) that uses one tube not sure how they do that, but Presonus makes a similar single path unit for about half the cost. http://www.platinum-records.com/Presonus-TubePre-Mono-tube-preamp-prod9488.htm

Another thread re:preamps {old-link-removed}

Here, check out this pre. I've got to get one soon, they have variable input impedance for your ribbon mic and are worth saving up your$ for, i've heard they are more versatile than the Grace:

http://pro-audio.musiciansfriend.com/product/Summit-Audio-2BA-221-Tube-Mic-and-Line-Module?sku=188005

I'm running into a Phonic Helic24 firewire board. I also have a TC Helicon Voice Live which I may or may not use. I was just under the impression that the Ribbon have very low output and need a good pre. I will try mine when I get it in but was just checking out possibilities.