I used to own two of GB128s ages ago in a cab that is now long gone and it was paired in a X pattern with two v30s. Recently a friend and I built two 4x12s and I since I had good memories of a GB128s, we just pulled a trigger on 8 of Gb128s to fill those two cabs.
It is - sort of - Eminence's take on a Greenback, but with a higher power rating. It does resemble the Greenback a bit, has that "woody" tone, and overall the speaker is very very warm! If you have shrill highs and icepicky highs with V30s - this speaker is much smoother. I could be said that it is on the darker side, but it is not. It is just that well balanced and lack any kind of high mid spikes in the 2kHz to 4kHz region that is known for various Celestion speakers.
Paired with a Hiwatt DR504 - perfect vintage woody tone with thick bass and strong mids. Paried with a Laney VH100R - copes with high gain great, does not fart out, stays tight, mids are there, highs are not over exaggerated! Ampeg V4 - excellent! Any amp we tried and A/B-ed them with an Orange v30 cab, it just rips and blows it away, much smoother tone, thicker lows, and excellent tone!
This is a sleeper speaker, goes under everybody's radar. Sensitivity is quite high - over 100db, and it delivers quite a punch! Best bang for the back in the guitar speaker realm. It is not a Greenback, but provides a bit modern take on a true classic.
Highly recommended!!!