Often misunderstood is that Loop Amplifier are not standard amplifier, they are current or trans-conductance amplifer. The output Impedance is usually 1ohm to a maximum 2ohms. I have installed system as low as 0.5ohms but care needs to be taken.
There is a new Australian Standards AS1428.5 that is required on any new building or one built after 2010.
To be effective only a single coil is used to reduce current distortion created by additional inductance. The current output is used to determine the size of the system usually with a minimum distance in one direction. 5m wide for 150sqm area, 8m wide for 500sqm. There are many limiting factors as you are producing a magnetic field metal used in concrete re-bar will sap away some of the emitted energy. Current output ranges from 2amps for counter loops, to 60amps for up to 2000sqm. The largest single installation was at a temporary open field rock concert at 1000sqm.
In addition AGC not ALC is required as peak power can destroy the output stage either instantaneously or overheat in time.
Finally as part of the International standards the field produced 1A/m.
In short, Loop amplifiers are not your normal everyday amplifier. Especially now-a-days with smaller an smaller hearing aids with telecoils making it more importantly to have high enough power level. Hearing aids have their own standards which is why they need to match when it comes to the designs and implementation for both.
If anyone is interested, I own and run a small business that deal with hearing loss products in W Australia, for 12 years I was installing loops up till 3 years ago
So I am not talking out of my arse
I stuffed up my knee crawling on the floor installing copper foil, so I no longer do loops anymore.