The difference between vintage warmer / other warming software fx and hardware is subtlety.
The soft plugs I've used (not TDM, all VST/AU) are all to my ears, very blatant effects. That or they just sound like algorithms. What I mean by "sounding like algorithms" is kinda hard to describe. Easiest way to put it is that they sound fake.
Modern flat sounding music isn't so much due to software- big budgets can afford to use whatever outboard they want. It's because (IMO) modern tech (hardware and software) to polish a track to such an extreme that it's dead.
High quality outboard gear can make a huge difference in your mixes. If I were you, I would do the following-
1. start with quality outboard fx i.e. eventide or kurzweil or other big $ fx platforms. do this because quality fx will make the greatest impact on the sound of your mixes. you'll hear the difference between a good verb and a mediocre/shit very in a mix much more easily than the difference between an $800 and $4000 A/D converter (unless you're dumping piles of analog sources into your mix, in which case the shite conversion may become noticable). Get GOOD fx. I picked up a eventide H8K (PRICEY) but it is absolutely amazing and made a huge difference in my mixes vs. when my mixes were based on plugs (and I'm a plug master

).
2. get a top end EQ. Color/clean, it doesn't really matter. Just get something that's stereo so you can use it for busses (including the master if necessary). This is of course, assuming that you like using eq. Some people don't. I do. Good hardware eq kills all but the best (so i've read about the gml oxford eq) software eq.
3. if you're really tracking a lot of outboard synth, get a good DI. The TAB-funkenwerk is awesome (i've got one) and it really is very very nice.
4. get some nice compressors! software compressors are weak and can't do nearly as much or sound nearly as good as hardware comps.
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If you really want your stuff to sound more 70's like, try the following-
use fewer tracks for your songs/mixes.
use fewer fx.
use less compression
don't smash your mixes
learn to engineer well (the old school engineers seemed to know how to make things sound GOOD much better than the new school does IMO)