from a cursory inspection, the instruments all look to have survived their overseas journey without major damage.
2 of the 3 basses have slightly mis-aligned necks and all three look like they will need to have their neck pockets shimmed in order to facilitate low (as I like to set it) action. I like setting to ~2mm. I figure if it's setup and plays well there, the action can be raised to taste without major changes to other settings. One of the basses has a wonky pickguard and the front pickup on one seems to be a bit off. I'll have to check if re-aligning the neck fixes it or if I'll have to dowel the holes and re-set the pickguard. SX has never been terribly good on the assembly end, but I guess that's because they are "custom handmade"

haa haa. . . by monkeys with hand drills that is. . . sometimes, I'd prefer if they were made by machines.
Since the middle one had a slight mis-alignment, I figured I'd start off with that one. A quick trip to the amp confirmed that both pickups were working and gave me a baseline feel for the tone of this particular instrument.
And, off I go to address the neck joint first:
I figured I would try to make it so I could shift the neck in the pocket a little towards the "top" of the instrument to close the gap on the long side of the neck pocket and straighten it out a bit. Because SX basses are screwed straight through both body and neck in one shot, I'd have to enlarge the holes on the body. Since that would theoretically make the joint a bit looser than when I started, I decided to try installing threaded inserts so that I end up with a stronger joint than I started with. Also, since the holes are not straight, I'll have to try and follow the existing holes with a hand drill. . . weeeee. . . exciting. I figure since these particular inserts only require a 15/64" hole, I could always drill out to 1/4", plug and start over if I completely muck it up.
Holes are drilled. . . it was a bit nerve racking
I carefully screwed the inserts in with a 4mm hex key. The first couple of turns are critical. I had to keep the inserts straight while twisting it in so that it threads in straight. I have a pretty steady hand, so eyeballing it worked for me, and I went slow.
I'm using 8-32 screws that are 1 3/4" long. I ended up cutting about 1/8-3/16" off of the screws because they stopped engaging the threads and would be useless after that. Shorter screws also gave me a bit more wiggle room in case I mucked up the inserts in the neck. These screws fit perfectly into the existing holes in SX basses, so if I did not have to shift the neck, I could just assemble at this point.
But, since I have to shift the neck, I need to enlarge the holes slightly so I have some wiggle room.
Inserts or not, I don't like taking the neck on and off a lot, so I figured I'd shim the neck now, and hopefully, it'll not have to come off for quite a long time because I think I'll level and re-crown the frets with the body attached. I used a business card.
I put the neck back on, and lucky for me, the screws found their way into the inserts. The slightly enlarged holes in the body did their job and I was able to shift the neck into position and then tighten it down.
Oh my god. . . the tone was different after the neck operation. The entire body felt more resonant and the notes rang out in a "bell-like" way. Attack and sustain were improved and the bass felt more lively, responsive, and even across the entire fretboard. THIS is a worthwhile mod. I would not have thought to do it unless the neck pocket was wonky like this one, but I will probably be doing this to all of the other ones. It was a little scary, but in the end, the install was not that difficult.
Sidenote: I bought 100 of these critters (inserts and 1 3/4" screws) and will not be doing 25 basses so if anyone is interested in buying some, very reasonably, please let me know. They are EZ Lok 800832-10 8/32 internal thread inserts and I have flat head stainless steel phillips head screws (because I couldn't find a good price on oval heads).
The neck position pickup still looks to be a bit mis-aligned. I'll see if I need to re-set the pickguard. I hope not, but I guess it's not that bad if I have to. If it is out, it's by a pretty small amount. That's all for now. Tomorrow I'll do the same for the other bass with wonky neck and move on to removing electronics and fret leveling. Amazingly, the action on this bass just happened to land at a hair over 2mm with the neck shim and the bridge untouched. Also, amazingly, the bass is quite playable with just few minor buzzes here and there with the action this low. I'm hoping I will not have to take much off to get this fretboard level. Of all the 9 SX basses I've seen lately (keepers and returns), this is a first for me. Most of the time, there are high and low frets all over causing buzzes unless you set the action pretty high.