Basswood is lighter than Alder and softer. To my ear it gives a tone more rolled off in highs than alder (which is rolled off but I LIKE the alder tone). Basswood in general has a kind of "foofy" sound to me in passive basses. And being softer and more absorbing one would expect reduced sustain as well. People love the light weight of basswood, but I love the brighter tone of ash even though all my Ash SX are pretty heavy. But happily, wood is a pretty minor effect.
A bigger effect is the weird way that the parts in SX 6er seem to change! The 3 and 1/2 I have seem to vary from one to another. The first two I bought seemed significantly better than the third one and forth one, with pickups having lower output and generally not being quite the same. As you say it's almost as if they are doing prototypes and selling them! But that said, I must say that like you I LOVE my SX 6ers (even though my main axe these days is a G&L L2500 (5er).) The choice of having three of them in Freless, rounds and flats is simply wallowing in bass luxury!
And now instead of exchanging my Ash SX body (natural) for the burst one on my SX with rounds, I'm gassing for finding an SX neck for it and doing the blocks conversion on it. Kind of redundant, I know, but like you I'm really taken with having three identical sunburst 6ers.
