I don't know if boomers have eq/level adjustments
Some high end ones have the means by which to adjust bias and eq for individual tapes, for example the B&O and my Technics have these on surface-mounted VRs so that a small screwdriver can be used to optimise these settings. Some other boxes have these determined by the values of the passives in the area of the line preamps and as such, adjusting involves desoldering and replacing, sometimes even rewriting a particular region of the board. I did that when I was modifying the Sharp GF-555 so that's now set up for SAs on T2 position and ADs on T1 position for the deck that records. Deck 1 was left o/e as it only plays and was already set pretty much where it needs to be.
Maybe some Aiwa kit has easier to adjust settings in the style of their separates decks? The ADF410 that I use must be about the least expensive machine to have been made available o/e with these adjustments on the panel. The only Nakamichi deck that I have is the one in my Hacker Sovereign RPC1 boombox, it's a quality thing but has none of the complexity of the Dragon, Hacker famously sourced the parts that weren't built in house from the biggest names of the time so that deck, the FM head from Gorler, the speakers from Celestion, Elac, Goodmans or Tannoy went into their products in both portables and system separates. They could have used Thorn, Sankyo or Matsushita and in reality, noone would have been any the wiser.