I have Triton Two's with two Forcefield 4's running from a Y splitter from LFE from receiver. Triton Two's subs set at 1:00 position. Room is 4,000+sq. ft.with 11 feet cathedrial ceiling. Sub out set at 80HZ. Speakers set to large- tried small but they sound better this way. 100% music 2.1 system. I like late night listening with Tritons low- but I still wanted visceral bass effects{I'm a drummer who wants to feel realistic kick drums-about 100db}

My question is how to set the volume of the Forcefield 4's?

I read the sub gain on the receiver should never be past zero or distortion will be added to the signal. Receiver gain goes up to +10. A friend who knows subs said +5 or +6 wouldn't add any distortion.

Another post said to put receiver sub out volume at Max and adjust volume from the subs. Which is correct?

I'm a bit confused by the setup, but your receiver's bass manager may do things differently. Typically, if you set L/R to large and sub to yes you will get LFE ONLY on the LFE (sub) out on the receiver. Since you say music use (presuming 2.0 source), the subs would never be playing in this scenario. Normally, Bass Management should be: Small and a 40-60Hz crossover on the L/R mains, sub Yes (and the same crossover if there's a separate setting).

Here is A (there are many) procedure for multi-sub setup:

Start with the subs vol off and receiver's sub volume at 0, put on some music that includes a wide range (especially bass and lower midrange). On one sub, slowly turn UP the volume until you can clearly hear the sub adding weight to the midrange (some describe this as "muddy"). Now turn the volume slowly DOWN until this muddiness just stops.

Go to the receiver's test tones and play the subwoofer, then measure the volume with your SPL meter at your listening position. Unplug that sub and turn up the other sub until you get the same SPL. Plug back in the second woofer. Now that both subs are playing the test tone will be louder, so using your receiver's speaker level setting, adjust the now-combined-output of the subs to the same SPL level. That's your starting point. Listen and adjust with receiver's sub level to taste.