I've considered doing this with a 48v phantom powered design... it would demand a mic pre (or someone making bass heads with XLR inputs with that voltage of phantom power), but it would solve a lot of the issues. or alternately a twin power/output cable so that the extra cable wouldn't get in the way of your playing but simply continue 1.5 meters beyond the actual jack/jack cable.
+ doing it in a seperate graphite wing on the bass, with rills in it for cooling and veneer wood for looks, thus isolating the tubes from any wood (nobody wants a bass accidentally catching fire right?), while at the same time giving you a bass with tube glow... +1
of course I'm not a bass builder or electronics engineer (yet! I wanna! u.u) but I think this would solve many of the inherent problems with a internal tube pre.