Well, consider that no matter how stiff the neck is, approximately 25% of the overall load bearing structure is the body. While most of the body is considerably thicker than the neck, the neck pocket is relatively thin. So you've got some compromise there in the overall stiffness of the structure.
It's not a very big difference, but it is there.
My Q6 is a neck through (these were called TBX models; the bolt-ons are called SPI's), and unlike every other bass I've ever played, you feel pratically no vibration in the body or neck when you play. Nearly all the energy of the vibrating string remains in the string. It's especially noticeable with the low B string - my 5's all resonate a lot more on the lowest notes. That means that some of the energy of the vibrating string is being transfered into the neck and body of the bass where it dissipates instead of remaining in the string to be sensed by the pickup. That's not necessarily a bad thing - the various frequencies that are absorbed and damped by the structure of the bass are what gives different woods and construction methods their signature tones.
With the stiffness of a structure that is all carbon fiber, practically no frequencies are damped or absorbed by the neck or body, so when you pluck a string it's "all there". Technically speaking, if you had an EQ with a bazillion ultra narrow bands, you could emulate practically any bass sound you wanted to. I recall reading that this was Ned Steinberger's original intention for his carbon fiber basses.
Anyway, my neck through Q6 is the deepest sounding bass I've ever played, and I think that is due to the ultra low frequencies not being damped by the structure of the neck or body. It has a depth and clarity that my Genesis 5 and my Dingwall AB1 do not. The AB1, with the extended scale length, does have really great focus to the pitch of the low notes, but it's still a different thing from the TBX. The Genesis has a resonance all it's own, too. I don't consider these basses to be any less than my TBX, just different. The TBX is fretless and the others are fretted, so aside from the different basic tonalities, they serve different purposes.
The neck throughs are extremely rare though, and from what I've heard Modulus has no intention of making them again. I've heard a few times that there were less than 100 made. So if you want one, it's gonna be a long search. They are different, but they're not that different, so I wouldn't go too far out of my way or pay a ridiculous price to get ahold of one. I just happened to run across this one on e-bay, and got it for what I felt was a reasonable price ($1900). It's loaded, coca bola top, Bart quad-coils and preamp, and in nearly perfect shape, so I think it was a fair deal.
