I'd been trying to get an Ion USB Tape2PC to play through an iMac using Audio Hijack, but that app doesn't monitor audio unless you're recording.
Just thought to check the Rogue Amoeba site to see if they make any other utilities and found Loopback which let me monitor the USB audio input. As a bonus, it let me name the source, so now it shows up in Audio Hijack as "Tape Deck" instead of "USB Audio CODEC"
Now playing: Massive Attack
Whoops. Loopback is $99 and does way more than I need. Turns out if you hold the Option key when you click the Sound menu bar, there's a super fast way to shift the native Input Device, i.e., monitor the tape deck or any other USB input audio. Ha!