I found that the subwoofer bass in Stereo, Bypass, and Upsample modes are basically the same. However, the bass becomes much louder and sounds better (deeper) when I switched to the 5.1 channel stereo. Why is there such a big difference in the bass from subwoofer between Stereo/Bypass/Upsample and the 5.1-channel Stereo modes?

(In the set up menu, I have my front L/R speakers (Dynaudio Contour S1.4) set to “Small”, crossover to 80 Hz, and Subwoofer to “L/R+sub” . There is no center speaker or surround speakers connected. The front speakers are connected to Outlaw 7500 power amp. Analog signals from Rotel RCD-1072 CD player are feed to 990 preamp. The LFM-1 subwoofer is connected the subwoofer out of 990. All cables were from BlueJeans.com. Balanced cables are used to connect Outlaw 7500 and 990).

I haven't spent much time messing with 5.1/7.1 Stereo, but I'll offer a theory based on my limited experience and understanding. The 5.1 Stereo mode has always been louder than normal - same signal is being copied around to other channels, so you have more speakers playing at the same time. When you've got 5.1 Stereo mode going with small speakers all around, you have five channels redirecting low frequency information to the sub rather than just two, so it makes sense for the sub to be more energetic. I'd be curious to know what you find if you go back to stereo or upsample and simply dial the subwoofer trim up four or five dB.

You have the mains set to small and the sub set to L/R+sub? I didn't think that was possible. The L/R+sub mode is supposed to be used with large mains (it's a form of "double bass" in which the mains are getting a full range signal but a copy of the data below their crossover point is also handed off to the sub to reinforce the low end).

iirc, you can get a combination of small mains and sub L/R+sub setting by:
- setting mains to large
- setting sub to L/R+sub
- setting mains to small

I was playing with my 990 main/sub settings a couple nights ago and came across this.

Rob

Interesting, but I don't think that doing that will yield a result any different than having the sub as simply "on" with the mains small - the sub still gets the data below the mains' crossover point, and the mains still don't.