A little more testing. Last night
1. Batteries were at 100 when I turned on according to Victon monitor
2. Turned furnace on
3. Turned on TV and DVD and inverter.
4. Not playing through stereo, things were stable
5. unplugged subwoofer, played DVD through stereo things were stable
6. Plugged in subwoofer, ...still stable!!
I could not reproduce tonight either.
I noted While under bed plugging/unplugging subwoofer that the input light was green on inverter. Checking manual this denotes that the line voltage is 12v
to 14v. According to victron, battery was 12.71 during load of furnace fan, lights, sub, tv, DVD, inverter, radio..all cranked. 12.77 with furnace only.
I don't know why I can't reproduce...