comparing the string spacing on my two Ibanez 7 string basses, the BTB7 string which has 15.5 mm spacing vs. the SRAS7 which has 16 mm spacing (which difference seems very tiny):
I have slight issue of fretting the higher notes of the SRAS7 fretless strings, especially the A string which located at the far left side of the neck (if we're facing the bass on standing position), due to the SRAS7 has a wider neck at the higher frets area than the BTB7, whereas the low-B string on the BTB7 is still quite reachable on its higher fret registers.
I found the 7 String BTB to be pretty comfortable anywhere after the 6th fret. My main problem is that, due to the spacing and scale length, certain chords suddenly became uncomfortable to play (too cramped), among other problems. Additionally, playing in the lower register was a pain (literally).
Shortly after I returned the BTB747, I purchased a Conklin GTBD-7. It is easily one of the most comfortable necks that I've played, especially for a 7 string. However, after doing some research, what really hit me is that I noticed many of the custom luthiers (Warwick especially) are making these instruments with a narrower nut width. Based on my experience with the Conklin, it makes it significantly easier to utilize the B and E string in the same way that you would on a 5 or 6 string. Granted, the Conklin having a 34" inch scale length makes a big difference, too. The experience was different from the BTB, and from the sound of it, the Fretless/Fretted hybrid that you mentioned; both of which pretty much maintain the same width all the way down.
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If I could compare, the guys at Fodera created this GORGEOUS 7 string bass that I've been obsessing over for the last couple of months. You can see what I mean about the nut and neck shape, and this instrument is a 35" inch scale bass with 16.5mm spacing (Still a bit too tight for me, but better than 15.5mm). The Conklin GTBD-7 is 17mm at the bridge.
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Interestingly enough, the Fodera also adds more frets and has the neck extended (Or rather, shifted) to the neck pickup. I really wish that I could see how it would feel in the hands, and I assume that adding more frets would somehow move them a little closer together since it still maintains the same scale length? I don't know, I'm just assuming.
To be honest, the BTB747 sounds much...
much better than the Conklin in my opinion, so I'm trying to get rid of it. I really want to purchase another BTB747 but, if I take the jump again, it has to look..."special". If I can find a top that looks as beautiful as my BTB 746 (or even somewhat close) then I would be super happy.
I'm hoping that my mind will change and that, suddenly, the 7 string BTB will be ultra comfortable lol. Honestly, I just need to find a good day to go to Guitar Center and see if they have one that I could test before purchasing again.