Is the lack of bass from being recorded on cassette or the source?
It's the source material. The problem with mix tapes is that you can get all these dissimilar recordings - from different periods of time and mix boards - that simply sound varied in EQ, and when you play them back on revealing systems - you can hear the big differences in the overall spectra across the different period tracks.
You can't just focus on recording level alone when dubbing to tape, matching EQ is just as important and purely (record) level matching doesn't tackle differences in the bottom end punch or the rest for that matter. Old school was to record through a tape-looped equalizer as you made the transfer. Never enjoyed doing that unless it was an entire recording - think of rough, live bootlegs - where it was pretty much 'set it and forget it' for the entire session.
On this particular deal 6 years ago, I spent more time getting rid of digital harness on tracks from MP3 sources and normalizing overall volume levels, not so much tweaking particular bass or mid-range freq ranges. With the newer software, stuff like this is more 'canned' and makes a world of difference with very little effort.