Never connect incompatible connections. Speaker outputs are for speaker inputs. Period.
The various inputs on this or most any amp are for Line Level signal with the exception of anything explicitly labeled "PHONO" as that would be for a non-preamplified turntable ONLY.
From what I have found online, Lab-In was a NAD feature that restored the frequencies that were rolled off when turntables were being used in the (assumed) preamplifier that fed this amp.
"NAD belleived at the time that practical PHONO stages and the preamp and power amplifier stages be rolled off at subsonic frequencies to prevent subsonic wow and flutter from Turntables and supersonic frequencies (EMI/RF/Radio) signals from getting into the system in the first place. Their notion was that if your ears cannot physically hear it, then why amplify it. For the PURIST, there were a set of LAB IN amplifier inputs with more extended response called "LAB IN" inputs"
Use the Lab In's UNLESS you have a turntable source.