It's hard to tell differences with the graph as they are aligned slightly differently and they look very similar.
The differences I could tell were possibly slightly better matching on the TH-900 (seen in impulse response and treble), the first spike up on the square waves is a bit sharper and higher on the TH-900. Hard to compare treble because of the matching, but the TH-900 may have a bit more sparkle.
The TH-900's bass 30Hz wave seems to slant slightly less and the first notch goes higher than the hump on the TH-900, where it is slightly lower than the hump on the TH-600 which seems to make a difference with the %THD+noise reading.
Impulse response is slightly better/faster on the TH-900. Looks like the TH-600 has a tiny bit more subbass, at least the frequency graph seems to bend up a bit more there. Slightly bit more of a dip around 2-4KHz on the TH-600, but hard to say as it depends a bit on the compensation applied.
Again, both look very similar to me. I haven't heard the TH-600 and I wrote this/tried to write something because I was bored.