Diesel jeans and too-many-Scottish-Breakfasts-gut more like!
"Could the lack of tone be due to lack of body mass? Are such basses a bit light in weight?"
All of the above. Flying Vs look mammoth, but are light, just like their guitar cousins (the basses don't have larger bodies btw, irrespective of maker). And, of course, the wood "comes in late", the main mass being in the extended wings, far away from where the neck meets the body. That is the (high) price you pay for that marvellous high register access (in the Dean's case: double octave, you can see in the above pic quite well how "forlornly" that long and rather thin neck sticks out of the body).
What makes Flying Vs preferred heavy metal equipment for guitarists is actually what makes them bad basses: That they sound inherently thin. That is great if you use ultra-distortion, hi-gain pups etc, lots of effects - a Flying V will then still deliver a non-mushy signal where a Les Paul would long be just a barrage of noise. And not look like a Telecaster (the guitar I find that it sounds most akin to). Likewise, if you are Rudolf Schenker and solely a rhythm guitarist and do not want to muddle up the overall sound with your playing. But who wants a thin sounding bass? There is a reason that among the legions of bass players, even the more flamboyant ones, there is no one who ever made these "their" instrument. JAE, Testsu, Casady, Lea (I can't think of anybody else), they all just played them for a comparatively short time before discarding them again.
My best sounding one is my Dean Razorback (not the above one which has a much thinner neck and is set neck and not neck-thru). And that achieves a full, if not wholly remarkable sound via the following "tricks":
- aggressive active electronics,
- neck-thru construction,
- 35" scale and a phat and rather wide neck lending mass where the body fails to.
