Different manufacturers have different settings for using the SUB with FRONT set at LARGE. If using a Denon/Marantz with FRONT set at LARGE, the SUB must be set to LFE+MAIN. If using an Onkyo/Pioneer, the "DOUBLE BASS" setting must be turned ON with FRONT set at LARGE to send a signal to the SUB. If using a Yamaha with FRONT set at LARGE, the "EXTRA BASS" feature must be turned ON to get a signal to the SUB. Never use "EXTRA BASS" with FRONT speakers set at SMALL as it adds a bass boost to all channels compared to the low frequency duplication when FRONT speakers are set at LARGE.
Keep in mind that using ALL CHANNEL STEREO mode in a low/mid tier receiver while setting all speakers at LARGE will result in that receiver running quite hot. Also, if speakers are set to SMALL, changing the mode to PURE AUDIO, PURE DIRECT and DIRECT will result in two channel audio being sent to the FRONT speakers in full range regardless of the SMALL setting. However, a signal will not be sent to the SUB while playing a two channel signal in those modes. Though, a signal would be sent to the SUB in STEREO and ALL CHANNEL STEREO modes with speakers set at LARGE with the proper SUB/BASS setting in the receiver.