Well, I have had owned the shuttle 6.0 and the wt405 (but not the 550), not that famaliar with GK. I have used them in rehearsal studios, but have not owned one.
vs the shuttle 6.0....
I find the mb warmer, with more low-end on tap and its capable of producing more overall volume vs the shuttle. The shuttle is more high-fi / modern, while the markbass achieves vintage/old school better-easier IMO/IME. Both great amps, but quite different sounding.
vs the wt405....
I havent owned the 405 in a number of years, however, I do find the MB warmer with a little more volume ability. The 405 has better features such as mute, 2 ohm capable and better di. However tonally, the LMII that I own is just more my sound, plus it weighs less than half of the 405.
Cant really comment vs GK, since I have not owned one.
The LM tube, it has been said, does not have a strong tube sound with the tube 'engaged'. Most says its subtle. However, it has also been said that the louder you play the more noticeable it is. The MB rep says that the reason the tube doesnt seem to be more than subtle is because the LM stuff sounds fairly close to tube already. Many have made that claim as well.
Overall, the best thing to do is go down to a music store and A/B them yourself. Or even just buy one from GC give it the full 30 days to try out and then return it if not fully satisfied. The only real way to find out if an amp works for *you* is to have it out on a number of gigs in rooms with different acoustic properties.
If you check my profile, you'll see that I have gone thru a number of quality amps and the LMII is by far my favourite for the tone I seek.... which is a warm yet still punchy, P-bass with flats type sound.
Hope that helps.