i'm familiar with tvheadend but i always used with DVB-T Network; just once with IPTV i made a little test using a m3u file. Anyway, let me know details about "some channel loaded as a test": do you mean using some URL? moreover, app like iptv-smarters generally are not useful because of their bad performance. perhaps i have hint for you to get your goal.
so, what is your target? load a list of FTA channel or load your own iptv subscription?
i've my own working tvh server, so i can do any kind of test on my machine.