Monior line is on v4 right now. V3 would be anywhere from 2-3 years old, v2 about 4 years and older. Sonically, they all blend fine AFAIK. I had a v3 370 with v2 5s and they sounded fine together.
For the Performance line, the changes are very minimal from V1 to current; mostly cosmetic. I owned v1, v2 and v3 at the same time of the Titans and they all sounded identical.