Hi Ridgy,
Well, it's a combination of the R75 design and the proximity of the flame throwers.
I had a quick scan of the LW band on my R2000 and on 261kHz I got an S7 signal of our local sports programme on 828kHz which was giving me S10+20dB - there were a few other weaker signal on 261kHz too. I guess there's something going on in the mixing process or distortion in the local oscillator waveform.
So I had a look at the schematic for the R2000 and the antenna filter circuit is quite basic - a bandpass filter from150kHz to 1MHz lets everything through so there's nothing to stop 828kHz wreaking havoc and every other station below 1MHz. If you tune above 1MHz then there's another filter 1MHz to 2MHz so there could well be more birdies of AM BC band stations above 1.7MHz.
My NRD515 is quite well behaved as they have alleviated the problem. Below 600kHz signals go through a low pass filter so no BC band can get through. Tune to the BC band and there is a preselector from 600kHz to 1.6MHz to peak up any weak station and it works well in keeping the flame throwers out. Above 1.6MHz the filters are quite narrow compared to the R2000, the first one is only 1.6 to 3MHz.
The Debeg 7313, a proper marine receiver, a rebadged Skanti R5000 designed by Siemens Germany, is totally different. A great deal of attention has be incorporated to keep unwanted signals out, from 60kHz right up to 30MHz there is a quite narrow tunable preselector which peaks up any signal very well, but as soon as you tune about 50kHz away, then you need to retune the preselector. This obeys the long-held idea that you should apply maximum selectivity before any amplification or frequency changing. The LF band is almost free from all birdies and stations in the wrong place, although I suspect my re-alignment some time ago was not the best as it requires an RF voltmeter which I didn't have at the time - but I do now so it may get a going over. The narrow preselector is essential if you are try to establish a true duplex phone call as your transmitter, may be 1kW, is only 300kHz away. Strangely enough, the good old FRG7 has a tunable preselector but the performance of the radio below 500kHz is abysmal! Shame!
Now your R75 is much the same as my R2000. A low pass 1.6MHz filter lets everything through followed by a 10dB attenuator - an attempt to reduce the cross-modulation and other nasties. Unfortunately if you have cut this attenuator out you have made life worse! You can switch in the front panel ATT to put in a 20dB attenuation but that's counter-productive. Above those LF's, you have a 1.6MHz high pass followed by a 2MHz low pass filter so effectively passing 1.6 to 2MHz frequencies, than above that it's much the same, 2 to 4, MHz, 4 to 8 MHz. etc up to 30MHz.
I'm afraid you're stuck with it, unless you can rotate your Wellbrook! Of course, if you see a Yaesu or Kenwood Antenna Tuner grab it with both hands. Although those little tuners are designed to match longwires and other things, if you feed them with 50ohm coax then they just revert to a bandpass tuned filter. Whether they will go down tp LF, I don't know - I feel an experiment coming on.....