I own a Deon 4308ci and I am a little confused with your statement recommending not to use LFE + Main. I acknowledge that setting speakers to small allows base management but Denon actually makes the opposite claim that you do regarding LFE + Main. Denon states "Select “LFE+Main” if you want the bass signals to always be produced from the subwoofer". If you select the LFE only option, it states that all base signals will be sent to the sub only if the speakers are set to small. It would seem then that there are two ways Denon recommends to get the base signals to the sub and not send the base also to the mains;
1) Set speakers to small and select LFE or;
2. Set speakers to small or large and select LFE and Main.
I get what you're trying to say, but I think that you have it backwards.
Denon is a very difficult foreign language to master and I would always be very wary of literally translating their manuals. The recognised Universal Authority on Denon translation is AVS member, batpig, who is the compiler of the Denon-English Dictionary and batpig has reviewed the FAQ answer (see below) and is happy with it, as indeed am I, who wrote it ;) jdsmoothie is another Denon expert authority and I see that he has also replied to you with the correct information too.
If you engage LFE+Main, you duplicate the bass which will be sent as a result to the mains and the subwoofer. This is wrong for all manner of reasons. There is only one way to send the bass to the correct place (the sub) and that is to set speakers to small and use the bass management provided in the AVR. If speakers are set to Large then no bass management will take place and the sub that you paid for will sit there doing nothing (other than handing the actual LFE channel itself (the .1 in 5.1).
Is perhaps the definition of 'LFE channel' causing some confusion?
The LFE channel is a separate channel that may or may not actually have any content in it - it exists mainly for historical reasons and it has nothing to do with the majority of the bass that you will hear in your system at home. Most of your bass will be mixed into the main channels and this is why you need bass management if you have a subwoofer - to send the bass that would otherwise go to the main channels to your sub instead. The sub is the best place for the bass because the sub has been specially designed to handle the bass and has its own powerful amplifier for that purpose - OTOH the main speakers will not have been designed to handle the lowest frequencies very well (or very loud). This is why AVRs have bass management in the first place so it makes little sense to bypass it altogether or to duplicate where the bass ends up.
Have another read of the FAQ answer below and it should all click into place - but the bottom line is this: you definitely do NOT want LFE+Main engaged if you want the best sound your system can produce.