Yes, the ET65 is not terribly efficient, and I find it does in fact have a magical ability to sound very good even at low volumes ... but does not give in to cone compression early.
The GB is kind of the opposite ... it will begin to compress and get loose after about 22-25 watts ... but it's also a fairly efficient speaker.
Maybe the combi would give you what you want ... but I'm just not sure! If you try it ... be SURE to post back here with your results!
Another good option would be the new "light" 12, the G12Q ... it might just be EXACTLY what you want...
Don't confuse effecency ... which is a measurement of the dB-SPL produced by 1-watt of 1k at 1 meter ...
with true volume in an amp ... unless you only plan to play a note somewhere around B (6) at one watt ... and the result will never be listened to at anything other than exactly 1-meter in FRONT of the speaker cone ... oh, and of course you can't use a cabinet ... and ... well, you get the idea!
The little G12Q would most likely have a tough time surviving your amps ... she's conservatively rated at 20-watts (True RMS). (The pre-production model I got fist was called 25-watts.)