Ok, so I found these 3 versions of a thunderbird body...
No disrespect to anyone that owns/likes the body... I'm not a fan of this body shape. So I was not familiar with the "Legend 4X" as I only recognize this body style as the Rex Brown... So I thought the X in "Legend 4X" was a typo for "C" (next key over) which would be the Classic - like mine...
Separately they all look the same to me.... except now that I see all 3 at the same time there are some minor differences...
Back in the 1980's when SPECTOR was part of Kramer, Nikki Sixx became an Endorsed Kramer/SPECTOR artist. He's always had a thing for Thunderbird style basses so Stuart built several including an "upside down" model that allowed access to the 19th-24th frets.
A major issue with me for the T-Bird design is that the neck is so far inset into the body that you simply CAN NOT reach these frets. If you're laying down the thunder in the low-end only, then you rarely come down below the 12th fret anyway. So these basses were designed for that type of playing.
Stuart moved away from that design once Kramer folded and honestly he didn't plan on looking back. But Nikki's "SPECTOR-Bird" basses became cult-classics... and each year Stuart would see more and more people come forward and ask for them.
Rex Brown and Stuart developed a close friendship when Rex was with Pantera and Rex had asked Stuart for a metal low-end bass with better access to the 19th-24th frets... Stuart looked at the old SPECTOR-Bird models and decided to simply take the massive lower horn and make it a cutaway. The Rex bass was born. It's basically a T-Bird with a single-cutaway design.
To my knowledge, Rex did play some ESP basses along-side his SPECTORs but he never LEFT SPECTOR and I've never seen his basses called a "Rox" except for one national online retailer... and I personally always figured it was a typo. R-E-X - R-O-X... one letter different.
In the time I spent with Stuart and PJ developing the SPECTOR website I NEVER ONCE heard either of them ever refer to the Rex bass as anything but the "Rex". So I'm not really sure that the "Rox" has anything to do with Rex Brown's choice of stage or studio instruments. His relationship with Stuart and PJ is that he sees them and hugs them like brothers. So I'm not so sure I'd put much stock in the Rox name being any sort of reaction to him playing an ESP. The Rex-Brown/SPECTOR relationship has ALWAYS been close by my understanding. Heck, he has three signature instruments... the Rex Bass, the RX-T Bass and the RX-GT Electric Guitar.
When PJ Rubal came up with the Forte models, affordable USA neck-thru models with "No Frills, All Tone", he added the "X" model (first in the Forte-4X and then the Forte-5X) to address all of the people who had asked for SPECTOR-Birds.
They Forte-X models were so well received that many potential purchasers asked for the P/J setup and wanted to know if they could get them in different colors, with neck inlays, etc... and after much debate, PJ talked Stuart into building and offering the NS-2X and the NS-4X. Because the only difference between these instruments and their traditional NS-Body siblings are the shape of the wings.
The Legend-4X was unveiled at this past January's NAMM show... I was one of the first people to play one. And it's the affordable SPECTOR-Bird option for those who don't want to purchase a USA model... There hasn't been any talk of a Euro X model as far as I know.
So that's what the history of the SPECTOR-Bird/Rex/X models is.
I agree with Trace... I'm not a fan. They're pretty beefy, heavy basses. I love the curved, comfortable shape of the NS-Body. And I don't like the limitation to the 19th-24th fret on the X-Models. The Rex Models address this with the cutaway, but again, they're not as comfortable to me as the NS-Bass.
Everyone has their own preferences/biases... and that's cool. No disrespect to anyone who loves the Rex and the X basses. Honestly if it has the SPECTOR logo on it, it's going to be awesome... For me, nothing gets the juices flowing like a sleek, comfortable neck-thru NS. It's the ultimate to me. Always has been.
It didn't make me hate the bass though...no big deal to shift the bass a little. The tonal just wasn't what I was looking for. At the time I didn't know about the Tone Pump adjustment or else I might have given that a tweak. 
I have a feeling down the road that I'll regret the day that I sold my Ian Hill sig,(I was given an offer I couldn't refuse and extremely high G.A.S. level) but...It's going to a fellow SPECTOR player and I'll be ordering a Forte 4 tomorrow! 
