The problem with these old TNCs is that many of them have not been updated for newer modes in years (The M7000 is a classic example). Even the venerable PK232 now has, in their newer versions, a soundcard input, probably in recognition of the many new modes that have come on the air of late.
You're far better off getting a soundcard driven software package than a dedicated TNC. However, for very specific apps such as FAX decoding, a TNC might do a better job due to the additional filtering some of them have.
It does depend on what you are trying to hear. For example, marine stuff can generally be copied with something along the lines of SeaTTY; for all the new ALE stuff that's out there, and coming out, PC-ALE (which is freeware) is a very inexpensive way to get your foot in the door without a lot of cost. You can later step up to software like SkySweeper (which in it's basic form, is still quite powerful, and not that expensive..).
Mike Chace-Ortiz's website has numerous links for just such software - the link for his site is in the Utility Monitoring wiki. Of course, this assumes you have a good desktop radio - you can get by with portables, but only to a point. Weaker stations, or those with interference, will quickly show the portable's limitations. Some of the better PC driven radios (like the RX320 from TenTec) are also quite good for digital decoding.
73s Mike