[G0HZU_JMR]

Looking at your youtube video it does look like there could be >20dB attenuation below 1.5MHz and this could be normal because of the T attenuator on this range. Probably best to check it out with a sig gen as you say.

Looking again at your circuit there is at least one series diode that still runs in a 50R section of the signal path. This diode is just ahead of the 9:1 transformer and will probably be the one most vulnerable to generating IMD distortion and it would ideally need to be a very good diode. This is one place where a suitable PIN diode could offer lower distortion although it would need to be a fairly exotic PIN diode with a long carrier lifetime as it would be exposed to LW and MW signals.

I did look at this stuff a few years ago and I did manage to find some chunky PIN diodes from Macom that worked quite well with very low distortion below 1MHz in a 50 ohm system. However, these diodes cost about £20 each.

I'm not sure PIN diodes would be needed for the preselector switching diodes because these diodes are switching 450 ohm impedance filters. The RF signal currents passing through these diodes will be a lot lower compared to a 50 ohm system. I'm not sure at what point the larger RF voltages (associated with a 450 ohm system) impact the IMD of the diode switches but in the past I noted a huge improvement when comparing the IMD in diodes in a 200 ohm system compared to diodes in a 50 ohm system.