Well, the HD 58X Jubilee, 600, 650/6XX, and 660S all share an enclosure and mounting system that is mostly the same. That will make a common influence on the tone for all those headphones, with mostly the driver and resonators to distinguish between them.
To add the HD 650/6XX character to what you described above, it has the most midbass and least treble, and this is evident in frequency response graphs as well as listening tests. Another thing about the HD 650/6XX is that the highs have a softer impact, though the micro detail level is still very high. It was initially designed as a stablemate with the HD 600, the HD 650 offering a less fatiguing signature for long listening sessions with still good bass presence at slightly lower volume settings, but still accurate enough that an audio engineer could get used to it and edit music with it.
In contrast to the HD 650, the HD 660S does sound more lively and agile, with more separation between notes. The HD 650 does still have a bit more midbass, and the HD 600 does have more treble and upper mids. I do still perceive the HD 660S as the highest technical performer of the series.
