My understanding about Boston Acoustics product lines is:
GT Line: overbuilt Class A/B amplifiers. Very good reviews. Could be a bit pricey.
GTA: The Boston Acoustics budget line of amplifiers that's affordable enough to compete with MB Quart and the like, but still has some features from the GT series (e.g. the Q-tune).
I am personally on the fence right now about buying a GTA-mono block vs a 4-channel PPI Phantom. If I knew for sure that I don't need rear fill, I'd go with the GTA mono block for sure based on nice reviews. However, I am still deciding whether down the road I could have some use for rear speakers with proper processing, etc (hence, the need for two more channels to amplify)