A year ago I was looking into tube preamps too, and the First Sound was very high on my list...second, actually. I ended up with a Lamm LL2 deluxe, and I'm VERY happy with it. The ergonomics (or lack thereoff!) are similar to the First Sound, and is also RCA only. I don't remember why the LS17 wasn't in my list.
RCA is not inherently inferior for all systems and all conditions. Sure if you have a very long IC and live in RFI/EMI hell, but do you? Mr Go, maker of the First Sound doesn't think there is a technical advantage per se; just depends on the implementation. So, do your amp and CD player sound equally as good through RCAs or XLRs?
In case you haven't seen them, Soundstage had a good series of reviews of approx $5k tube preamps around year 2000. They were helpful to me as they compare units.
Are you buying new? If so, then try to audition.
I was buying used, so I took my time to search the virtual systems and forums to find people with these preamps and a similar amp/setup to mine, and got in touch. I even found a person who had the same amp and speakers I have! It takes a lot of time, but has been worth it for me.
My two cents