Don't get caught up in the concept of needing 10 times the power to 'double' the volume. The concept of doubling the volume is kind of a red herring. A bit more power can result in a VERY noticeable increase in volume if the cab can use the power. The idea that somehow 'doubling the volume' is in any way the mimimum threshold for 'a meaningful increase in volume' is quite misleading.
However, most of the impact of increased power and the ability of a power supply to deliver clean, open power increases at transient peaks is very real, and a bit more power can be the difference between a compressed low end and a real, deep, open low end. If you get a couple/few db increase in overall volume while also obtaining an open low end, then that is just a bonus.
One of the signature tones of the old TF550b was its relatively underspec'd power supply that resulted in that compressed, tight, low end that was part of that amp's signature tone. The TF750's primary difference is a larger power supply that doesn't clamp as much as the previous designs, which is why the TF750's actual max volume (and power rating at 4ohms) is similar to the old TF550, but the low end is 'bigger and less compressed' on the low end (which interestingly, some TF fans didn't dig versus the more mid present and tight TF550).
Regarding the weight of the DB750, and also the significant more 'slam' down low, that is a direct result of the HUGE toriodal power supply in the DB750. Massive power supply that allows for very open, non compressed low end at high volumes. The 'hybrid' doesn't really add size or weight versus the pure SS (e.g., the 4 pound GK MB500Fusion has three 12ax7's in the preamp section).
Again, all good stuff which sounds and performs quite diffently.
IMO and lots of IME on this one