Tried Bel Canto? They do not have the ringing sonority of a good solo set up in thumb position, but they don't get muffled and respond very well all over. Actually, I think they'll be my string of choice for some upcoming audition; I'll tell you within a month whether they worked out or not.
They make my bass sound bassy in a good way, even in TP. I hope that this is what the orchestras are looking for; nothing equilibristic, just rock steady. I actually skipped a recent audition because it was asked by the orchestra that you'd play in solo tuning. I am against that too... but the solo pieces I think are no big deal. The solo part is easy, just prove that you can manage a variety of styles and make a good interpretation, and that you have mastered the whole range of the instrument; the difficult part is the excerpts, where for some reason I can't prove my worth any more.
The concertos are actually a bliss to us young players, if you ask me. Takes a lot of experience to play an excerpt; the concertos is our chance to shine and at least get us onto the stand-in rooster where, in the end, we might have acquired enough orchestral experience to deliver what the orchestra wants to hear. Because, you can't learn on your own how to sound good in a section!
(At least I can't, but of course there are other people out there who won every audition they ever did from the year they were 19 and on.)
Edit: "The ideal sound would be one that depicts the true character of the double bass."
I think you summed it all nicely up there. Use the strings (and play in the way) that makes you sound like a good BASS player.