594 KHZ is consistently clearer at night at my location in Essex. All of those LF channels are clear during the day apart from 558 right up to 648. I believe 594 was the first LF channel to be tested on from the Ross revenge for the second service before they moved down to 585 then settled on 576 until laser went off the air but that used to be bad at night in those days.
I would have though a midband upwards MW channel would work best for relatively low power and smaller antenna as antenna efficiency would be higher? I know the LF channels always go further overall but often the signal strength is lower at similar power so it appears weaker.
I heard Flashback at my location in Essex last week on 765 with pretty good signal strength but haven't caught it this time round on 576 but previously when it was on that channel it was weaker. I'm in North Essex.