Frankly, the 250 and 500 settings do nothing for me, but Jack doesn't use them either. They might be interesting for people that want to drive their extreme effects with a beefed up signal, other than that it is hard to see what there is to like about sounds that take all the nuances out of this nuanced bass.
I've been messing around with mine for a bit now, and I'm leaning towards the 250 setting. The 500 setting is almost too nasal and pressure-cooked. It is nice to have the 50 to roll back to, and the 500 setting to boost to. This will work for me live, as I don't like having to dance through effects pedals for different gain settings. If nothing else the design is useful, even if unwittingly so. Rather than just being three volume settings the EQ changes for each setting. The 50 has a nice wide bandwidth, almost scooped in the mids. The 250 setting is fuller and more present in the mids. The 500 setting has that honk to it. Oddly enough I sound like Jack Casady when I play this bass.
I'm quite pleased that mine is a Peerless example. It doesn't have that plastic feel I associate, perhaps unfairly, with Chinese instruments. The rosewood of the fretboard is a nice piece, the finish came back from the dead very nicely and the frets are level and hard! I had to do a subtle leveling of them to remove some roundwound wear on the lowest two frets, and the frets were otherwise level up the neck. They were a joy to work on.
This is mine now:
No 'E' on the pickguard and no poker chip under the 3-way switch.
I'm mildly tempted to put a Strat output jack plate on the bass just to strengthen the output jack area and to get the cable up and away from the top of the bass as quickly as possible. Blasphemy, of course, but I did the same to my much modified Aria TAB-66:
On the Casady the jack would be pointing up the way, rather than down. I've also since de-fretted this Aria, and clearly need to learn to leave stuff alone.