I'm rather surprised that you did not see the improvement to the HF performance that the upgrade kit makes to the original VLF/HF amplifier. It may be worth trying the antenna again but in a slightly different location.
The signal level from a small loop at HF is very small and needs amplification with a very low noise amplifier. The outer shield of the coaxial cable connected to the loop can act as a very effective HF antenna and produce a very strong signal at the amplifier output socket many dB stronger than that appearing at the loop.
A good common mode filter is required at the amplifier output socket to stop the high RF level bypassing the amplifier and getting into the amplifier input.
One test of how well this works is to look at the depth of the nulls of the loop. As others have already commented a loop antenna made with the Loop Antenna Amplifier gives very deep nulls.
Another possibility for the MLA-30 to appear to have a better performance is if the signal appearing on the coax is totally overpowering the wanted signal from the loop. In that case you don't have a loop antenna you have a long wire antenna!