You shouldn't have to change the balun on the board because it's flexible down below 160m - receives fine there. Just make sure there are no solder bridges when you removed the MN18 coil from the board...I had that happen to me before I powered it, and I corrected it.
I've been noticing, as there are others too, that there is an issue where the receive below 8 MHz is weak for some reason in GQRX, yet when I run by LimeSDR on SDRConsole over the entire band the receive is fine - perfectly normal. There was a thread I was trying to look up where another LimeSDR user was seeing the same thing but I cannot find it immediately - so bear with me on that.
@csete: LY2SS has an issue where his receive is fine on GQRX at 20m, but when he runs it down to 1 - 5 MHz the receive isn't that great. He's performed the MN18 mod on the board and is still seeing this issue. I, too, have seen the receive fall off below 8 MHz on GQRX. Not sure what to do on this, but keep us advised.
73 de Marty, KN0CK