The specific string to add to the device string when you set up which rtl_tcp instance to talk to is:
(0 = no direct sampling; 1 = direct sampling on I, 2 = direct sampling on Q.)
I was able to get it working well with gqrx on Linux but there were a few quirks.
1. You use direct_samp=3 in the options; I found this randomly on the gqrx GitHub page.
2. You need to restart gqrx once you change the mode, otherwise the reception is terrible. I think it's a bug.
Once you do that, you can tune in to the 0-24Mhz region and it works pretty well.