0 on the meters is -18 in digital. There is nothing wrong with this at all, it’s just very quiet compared to commercial CDs which tend to be on digital 0 for the entire track. []
You can normalise the tracks later in a DAW to get the level up, but -18 is about right for the recording.
What kind of levels do you see when you (p)AFL the LR mix? You said the faders are around unity but if you have your input gains very low this could be the issue.
Low levels shouldn’t make anything sound hollow, thats more often caused by phasing issues between similar signals.
Chris