I own none of their crafts, but some that come to mind are: Bill Conklin (Conklin Guitars) Jerzy Drozd (self-titled) Fred Bolton (Bee Basses) Adler Guitars Sebastien Aquilina (Aquilina Guitars) Ryan Welcome (Sage Guitars) Ergo EUB Instruments
Chris Stambaugh's attention to detail, willingness to give you nearly whatever you're willing to pay(a very reasonable price)for & ability to match the customer's requests w/construction, material & electronics choices is amazing. He knocked out a fretless 7 w/a bewildering array of options, made it work flawlessly & did it at again, an unheard-of price.
Listerud and ACG are both excellent choices. I'd also suggest FBB but be aware that his build schedule is really jammed up and he is pretty selective about accepting new commissions .... You might want to check out the Extended Range Bassist forum. There is a lot of information there pertaining to luthiers working on 7+ string basses .....
+1. Martin Petersen of Sei basses has made a number of very tasty 7 string basses in addition to my SEI 8 string fretless and the 9 string he is currently making for me.
This won't help you, but one of the nicest 7's I've seen and heard is my bass teacher's Curbow petite 7 in wenge. A very early Greg Curow build. Awesome bass.
Fred Bolton's works, apart from perhaps Jerzy's, are among the finest I have ever seen. I think my dream bass is yours in fact, James, but perhaps in a physical form more similar to Paul Delano's signature. That is a perfect bass to me. From what I've heard of your material, your tone is not of this world. Any more luthiers you guys can think of?