'in the amps'? If you mean the amplifier, that does play a big role. Be more scientific with what you are trying to hear and how you will be using it.
Let's take frequency response of 4 different loops
- MFJ-1886 500 Khz-30 Mhz
- Wellbrook ALA 1530 30 khz-30 Mhz
- W6LVP 135 Khz-30 Mhz
- DX Engineering / Pixel Pro-1B 100Khz-30 Mhz (down to 50Khz with reduced specs)
Now if you happen to be someone who wants to play in the basement (in the new LW allocations), the MFJ is out.
Want to go portable? I understand the Pixel needs 24 vdc which might be harder to do. W6LVP has a version that takes 8 AA batteries which is far and away easier to do for portable work
How much noise is produced by the amp itself? Definitely a question to find out. Remember gain isn't really the goal of using a loop - a better signal to noise ratio is. Also consider how well the amp behaves in a strong signal enviornment.
What about signal rejection - how much does the signal get attenuated when the loop isn't pointed in that direction? That's not a characteristic of the amp, more the antenna design. Obviously the higher the rejection the better. This becomes important if you're a MW or LW DXer, but loops can still have some directivity even if you're in the 160 meter band. Keep in mind that above 2 or 3 Mhz, skywave propagation starts to predominate, and a loop can have difficulty nulling one signal from another.
There's much to consider with using a loop. As the old knight in one of the Raiders of the Lost Ark movies once said...
Choose Wisely
Mike