Originally Posted by Yahzi
Also just to add, normally if there was a double bass function there wouldn't be a crossover setting. It would just send the bass from the mains (if set to Large) to the sub. So I really don't get how he has 40 Hz low-pass for the sub if all speakers are already set to Large.
The "double bass" or in the case of the Rotel, "Max" setting there still has to be half of a crossover, even though all speakers are large/full range. The half that is working is the LPF that feeds the sub. You wouldn't normally want full range audio going to a sub. In this, and most setups, there are two possible sub LPF functions, the one in the AVR/Pre-pro and the one in the sub itself. Ideally, you'd want to only use one of the two and completely bypass the other.
Even though "Large" or "Full range" speakers may have some extended bass, they aren't subs and can't hit the required maximum SPL at 20Hz, for example. Yet they may do ok between 60 and 40Hz, depending on the design. Hitting them with full range and allowing whatever energy they product below the crossover does two things. 1, it augments a possibly wimpy sub adding to the total maximum SPL, and 2, because the LCRs are in different locations, they could possibly even out bass response in a room that may otherwise be uneven because of modal distribution. That's the up-side. The down-side is that full range speakers usually can't produce high SPLs a sub frequencies without distortion. So, while you get additional bass, it's probably not very clean, or nearly as deeply extended. It's a compromise, sometimes good, sometimes not.
Why pick 40Hz for the sub only? Depending on the bass response of the main speakers, if they are really "large" with lots of woofer cone area and excursion in a cabinet designed to do some real bass, then they may well extend town to 40Hz and below. But below 40Hz almost every full range speaker runs into issues where it needs help from a real sub. Frankly, I'd have picked 80Hz, but I don't know the speakers.