Bad crossover in receiver or sub issue?
I could use some help with my new monoprice M15-s subwoofers. I've got an onkyo TX-RZ830 receiver, and i'm using Polk LSIM speakers (707/706c up front, and 702 surrounds). I'm noticing a distinct lack of bass when I listen in stereo (2.1) mode on my receiver. Last night I listened to a bunch of different songs on pandora and switched between DIRECT mode (pure 2.0 channel) and STEREO (2.1 where the crossover and subs kick in). The bass was much stronger using the DIRECT mode without the subs. I suspect an issue with my bass management or crossover setup in my receiver. If you all agree, then I'll go poke on the onkyo thread.
-Each sub is connected with a single RCA cable (receiver has dual sub outputs). I'm not using a Y-splitter to feed both left and right channels
-Both subs are setup the same: Crossover Max at 160hz, Phase Min at 0 deg, Level THX Ref. Crossover toggle off, EQ toggle extended, Power toggle auto
-Receiver has the crossover points on the front left/right set to 80hz (THX). The accuEQ wanted to set them to full, but I intentionally changed it back to 80hz to allow the subs to be more active
-Subwoofer level was set to -5.0db by the AccuEQ, but I adjusted it up +3db to -2.0 db
-nVidia shield (2019) was the streaming device I was using for these music tests (pandora app)
-To get a similar level of bass in STEREO (2.1) mode as I get from DIRECT (pure 2.0 mode) I have to:
A.) Increase the subwoofer level in the receiver to +8.0db (keeping the level on the subs set to THX REF)
B.) Increase the level on the subs to around +8-12db (while keeping the subwoofer level on the receiver at -2.0 db)
Whats really odd to me, is that when watching 4K blurays, or streaming something with 5.1 (dedicated .1 channel), I don't notice this lack of bass. In fact these subs really kick butt and are shaking the hell out of everything in my room during movies. I've also taken a bunch of REW measurements with the sub connected directly to my HTPC (not going thru receiver at all) and get reasonable curves.
Because of all of the above, I believe the subs themselves are in fact working properly (plus I have 2, what would be the odds of getting 2 bad ones). And I think that there is something wrong with my onkyo receiver or my settings with regard to crossover. I've seen numerous posts in this thread about folks thinking they have "low output" from their monoprice subs, thats the only reason why I question if there still could be something wrong with the subs... (altho those cases are usually folks with yamaha receivers).
If it helps, I do have an old Polk PSW505 sub around somewhere I could hook up instead and do the same music test. I also have an older onkyo non-4k receiver (HT-RC270) I could try to hook up. I was trying to avoid rewiring and swapping out gear, but if thats what it takes I'm open to all troubleshooting suggestions! Thanks for everyones help in advance.
System: LG OLED65C8PUA, Onkyo TX-RZ830, Monoprice Monolith 7x200
Speakers: 7.2 config: Polk 2xLSIM707, LSIM706c, 2xLSIM702f/x, 2xLSIM703, 2xMonoprice M15-S
Devices: Nvidia Shield TV 2019, Microsoft Xbox One X, LG UBK90 UHD, Win7 MediaCenter HTPC, HD Homerun Prime Network Tuner