The Yamaha’s Zone 2 will be a standard line-level output. So, not sure why the audio coming in to the TASCAM is low, unless you don’t have the settings right. The switch on the top should be set for Mic/Line, and the Mode switch on the bottom should be set for Stereo, and you should be using the two 1/4" inputs (next to the XLR mic jacks). The TASCAM also has input gain and output level controls, which combined should be able to get you more signal level than you actually need.
Regards,
Wayne A. Pflughaupt