Major changes from last gen (T1.2), inferred from information available online and auditioning the headphone first hand:
- 32 ohm driver with a redesigned coil former, which places the voice coil deeper in the magnet gap.
- Thermoformed foam absorber over the driver and baffle, which effects a broadband reduction in the treble response.
There may be other, undisclosed tweaks done to the driver or pads which would not be possible for a layperson to determine. Beyer has not been open about changes introduced with the 3rd generation, including those above. The product description mentions only an increased bass response, without referencing any specific technologies or tweaks.
In terms of gross frequency response, the new version is darker and punchier, with less air. Whether this constitutes an "upgrade" will need to be evaluated, according to personal preference and gear, on a case-by-case basis. I certainly don't see it as a "must buy" for all T1.2 owners.
By far the biggest acoustic change is the foam treble absorber. While not a new concept, particularly when it comes to modding, this is the first time Beyer has made an overt move to significantly reduce the treble response of any of their headphones, as far as I know. It'll be interesting to see if this is a feature they trickle down to other product lines. It certainly seems to be effective, as my impression of the 8kHz treble peak now puts it about in line with the highest level in the bass registers--though it should be noted, all frequencies above 4-5kHz seem to be effected by this reduction, not just the peak.
The most controversial change is obviously the impedance. The 32 ohm driver does seem to be revealing, but also sensitive to gear. So, contrary to popular opinion, these are by no means "easy" headphones to drive compared to the 600 ohm version. The tonal character of your chain WILL have a major impact on the sound of this headphone, so careful attention to synergy will still be needed to get the best result according to sonic preferences.
I think the new tuning of the T1.3 will be somewhat polarizing among T1.2 owners, due to the darker tonality. Some will appreciate the warmer sound, while others will be unsatisfied with the loss of top octave air. Some may welcome the extra bass punch, while others will find it overdone. And obviously, a system optimized for the 600 ohm driver will likely need to be re-evaluted to get the best performance out of the 32 ohm version.