> My next question is what are people's favourite receivers for use at 
> 500kHz? 
>

Hello Mike, LF

I use an AR7030, as others have recommended. Whilst the user interface
might be a bit maddening to some folks I find it okay; too many years
driving menu driven test gear at work? 

In fact the 7030 is effectively a piece of test gear. I value its
frequency stability and the repeatability of signal strength
measurements. It also serves as the measurement section of my ERP
verification field strength metering system. 

I recently added the Collins 300Hz mechanical CW filter which has
greatly enhanced its performance with weak signals in the presence of
high levels of QRN.

The sensitivity of the AR7030 is flat right down almost to DC, unlike
most amateur transceivers, so no mods are necessary to get the best out
of it an MF and the excellnt IP performace means no BC band intermod.

I built a tunable pre-amp recently which is only of use under very
quiet daytime conditions, when external band noise is low and a few
extra dB of sensitivity can be useful. 

I have a very quiet rural location and during the day can probably use
the extra sensitivity.

Many other 500kHz stations are limited by an elevated urban noise floor.

I have yet to need the pre-amp on 500kHz at night due to the much higher
levels of external noise but during the day it has allowed reception of
NDBs that were below the receiver noise floor indicating that the 7030
might not be as sensitive as it could be. 

However,IP performance is generally more important the noise figure on
HF though, so I see why the gain/IP compromise has favoured linearity.

Regards,

John GM4SLV