Update!
Curiosity took over so I quickly hooked up my T/T (setting it on the floor!). At -20db the volume levels were noticeably lower than most CD's and DVD's. I got curious and pulled out the dual trace 'scope and turned up the volume while watching for any clipping of the output waveform. It got pretty loud when I stopped. My vinyl is mostly stored away so I didn't try a test record. I think it's safe to say the volume differences aren't of any consequence.
BTW, although my main amp has more power output than yours that shouldn't make any difference in this case. I was looking for visible distortion in the 990 output waveform, not the amp output. I realize with music it can be very difficult to tell when amp saturation just begins. I had the volume up to -5 and it was plenty loud...the floor was moving. I wish I hadn't sold my last SoundTech 1700. I could be a lot more helpful with this. But I really believe you are okay cranking it up higher than -20.
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Outlaw 990; Carver TFM45 (mains); Carver TFM35 (surrounds); Carver TFM24 (bridged for center); Carver TFM15CB (rears); Acoustic Research AR9 mains; Polk CS400I center; Polk FX500I surrounds; Polk FX300I rears; Sony KDS-60A2000; Oppo DV-981HD; MX-850