Would the Active Antenna Amplifier (model AAA-1C) make a good masthead amplifier for a VLF loop antenna (SIDs)?

https://active-antenna.eu/amplifier-kit

Yes — the is actually a very good choice for VLF loop work and SID detection, much better suited than the LA-200 type devices.

The important point is that this amplifier was specifically designed for:

• small magnetic receiving loops
• LF through HF reception
• low noise
• high dynamic range
• balanced operation for common-mode noise rejection

Its specified usable range is 20 kHz to 55 MHz, which puts it directly into the SID/VLF region.

For SID monitoring around:

• 18.3 kHz
• 19.6 kHz
• 20.9 kHz
• 21.4 kHz
• 24.0 kHz
• 24.8 kHz

…it is technically well suited.

What makes it especially attractive for your use case:

• Excellent overload handling
  Urban VLF environments are full of strong LF/MF broadcast signals and switch-mode noise. The LZ1AQ design has unusually high dynamic range and good IMD performance.
• Balanced magnetic loop input
  This helps suppress electric-field noise from mains wiring, LEDs, Ethernet, and Raspberry Pi systems — critical for SID work.
• Works well with physically small loops
  You do not need a giant antenna for SID. A 1 m to 2 m loop is practical.
• Flat broadband response
  Useful if you want to monitor several transmitters simultaneously with Spectrum Lab or SuperSID.
• Proven by the SDR community
  The LZ1AQ amplifiers are widely respected among LF/HF receive enthusiasts and SDR users.

For SID detection specifically, I would recommend:

Best configuration

Loop

• 1–2 m square or circular loop
• Copper tube, coax braid, or parallel wires
• Single turn preferred for broadband SID work

Amplifier

• LZ1AQ current-mode loop input

Receiver

• SDRplay
• Airspy HF+
• sound card + Spectrum Lab
• SuperSID setup

Best installation practice

• Put amplifier at loop
• Use shielded CAT5 as intended
• Battery power if possible
• Place loop outdoors and away from buildings

The amplifier’s common-mode rejection is one of its strongest features. That matters enormously in modern UK suburban noise environments.

One caution:

For pure SID work, a resonant tuned loop can sometimes outperform a broadband active loop because it rejects out-of-band noise better. But broadband active loops are vastly more convenient if:

• you monitor multiple VLF stations
• you use SDR waterfalls
• you want continuous spectral coverage
• you also use the antenna for LF/HF listening

So overall:

The only thing I would avoid is using very high gain settings initially. With SID systems, overload is often a bigger problem than insufficient sensitivity.

A practical starting point for you would be:

• 1.5 m square loop
• single turn
• mounted vertically
• LZ1AQ amplifier at feedpoint
• SDRplay into Spectrum Lab

That would make a very capable SID and VLF observatory system.