Hello jaco523,
The M50v3 has been significantly improved over the old M50ti, with a newer and better tweeter. The old M50ti was pleasant enough, but in blind listening tests I found it rather on the "dull" side in the treble region -- not as detailed as the original M22ti, the M60ti or the M80ti. In recent listening tests, I was surprised at the improvements in the bass/treble balance of the new M50v3.
The QS4s should be OK. You should certainly check that the amplifier of the EP350 subwoofer is working properly before you lay down your money.
Generally speaking, the changes to the original "ti" versions with the subsequent v2 and v3 versions involved improved tweeters with better dispersion and power handling, adjustments to the crossovers for smoother spectral performance, and better drivers (woofers and midranges, where applicable).
Regards,
Alan
