Paulownia: It's very light and very soft. It's not as light and soft as balsa, but it's definitely going in that direction. Softer than obeche, which is another light wood I'm going to experiment with. As for paulownia, you have to be careful with it. It's quite grainy and porous in its raw state, so you need to grain-fill it and apply sanding sealer before sanding and priming. It is very easy to work with apart from that. I'd recommend a final coat of something very robust and hard-wearing to protect it. A marine coating, or maybe Ronseal Hardglaze, something like that.
I really like it, the disadvantages are worth it because it's so light. As far as sound goes, that's a moot point - in my opinion whether you use an exotic hardwood from the unobtainium tree, or a two by four from Wickes, it makes little or no difference to your 'tone'. This is a very short sound clip I made just after I finished the bass. I think you'd be hard pressed to identify which 'tonewood' this is, even though there are those who claim to be able to do so!