I suppose that's a thing that'll be highly dependent upon the specific rig. I'm glad you got yours going. I agree that there's nothing wrong with 80's rigs: my favorite HF rig is the Alda 103A (1979). Internally, those 80's rigs were pretty hard-wired to a unique design. The Alda starts out with a 9 MHz BFO, puts that through a 9 MHz mechanical filter, mixes that with a 5 MHz VFO, mixes that with an 11 MHz or 18 MHz oscillator, depending on band, and then puts that through the band pass and PA. How is your rig set up?
I'm going to look at it more closely to see what would be required for a lowfer output. Should be doable, like you said, far ahead of the filtering and the amps. In the Alda case, there's a LPF before the 11/18 MHz oscillator, which pushes the take-off point back a little. The prior circuits may not be wide banded enough tho ... but interesting to see ...
I think that in the future, SDR may be the way most hams will do lowfer stuff. Also, use can be made of those new synth chips (Si5351A, Si570, etc).