Some people do use cassette to record, and you might get good results with that. That said, digital is impartial. If your mixes seem empty, it's probably because of what you're feeding it.
If it were me I'd try an analog mixer with mic preamps that bring some of that warmth in. Or perhaps some classic microphones plugged into a small tube pre, for instance the ART tube pre, unless you have thousands of dollars to throw an an old Neve console, the brand which was at the heart of some of the studios where classic rock albums were recorded.
I personally like some old 60s Sony mics and a perhaps even older Shure PA system mic to add a soft, vintage sound signature.