Set your speakers to "Small" and set a crossover of at least 80hz. But understand if you do this and you don't have a sub, you're effectively killing off the under 75hz region of your main L/R/C speakers bass performance. Therefore, things will absolutely sound tinny.
The other way; get a 2 channel (or multi channel if you want) power amp and use that to power your main front speakers. You can then run them at "Large" i.e Full Range. However, the receiver or pre/pro will need to have pre-outs in order to achieve this, the v681 doesn't have pre-outs unfortunately.
Simply setting the volume of the sub to 50% (or 12' oclock) doesn't guarantee that the sub will sound loud. Why?
Room Modes, that's why.
To put it simply;
1) Maybe your sitting position may be in a HUGE null spot, that's why you don't hear the sub much.
2) Maybe your sub is placed wrongly in accordance to your main listening position, there by again - being placed in a null spot.
3) 50% is not a standard volume to set for all subs. Sub A can be 50% and hit 75db standard from the main listening position, however sub B might have to be set to 75% in order to hit 75db spl.
There are a multitude amount of other factors. This is why it's highly recommended to get an SPL meter and VERIFY the volume output from your listening position.
As for crossover setting, 80hz is a good general starting point. But it's not a set rule where all HT setups must abide to. It depends on the room and it certainly depends on the speakers. If your speakers cannot play down to 80hz with authority, then setting the crossover to 80hz will leave you with an obvious gap from the speaker's roll off -6db point. There is no "best" crossover setting, unless everyone owns the same speaker, sub and live in the same room.
What receiver are you using anyways?
Yup very true bro once I change to small and set the crossover to 80hz the bass is off...now my L/R bookshelf set to large and center set to small..now the boom from center speaker is off..now the sounds abit good
yamaha 681 can see the eq graph result from avr?
see if u ypao + loud volume vs pure direct + loud volume
who has more bass or not.
i worry the ypao cut down bass becos wall reflection.
use ypao flat, natural, front align. are different if u hass free time to test
I prefer ypao natural..ypao front feel like more bass..i run ypao again after the correction now sounds a bit good...yup bro got graph there I didn't tweak it caused no idea will take photo later