My personal tries with tapes were not successful, the result was not worth the hassle, and although the signal was compressed, there was a lack on certain frequencies.
I got much better results with saturation plugins (e.g. the one in Ableton) and loading samples into devices that have a typical 12bit compression like old EMUs.
If you really want to try, then I would suggest trying with a Revox... (Don't know if it is worth the investment).
I am aware of the restricted frequency range and it is welcomed, in fact, the 4 track machine I looked at recently on EBay ( I don't even need a 4 track, a single track would suffice ) specified a 40hz to 12.5khz frequency range - although I didn't quite understand this as I thought it would be the recording medium and tape type rather than the machine itself which dictated the bandwidth.....
If you've ever listened to Boards Of Canada stuff, there's a warmth and lo-fi quality to a lot of the synth parts, I also like some Legowelt tracks and I know for certain a lot of his synth lines and mixes get their 'aged character' from a tascam 4 track, basically recording onto the cassette and then recording the tapes output back in the computer.
I've also looked into replicating the effect digitally and for say a synth line ( say some pad chords ) if you restrict high end information and cut off at about 12khz, you immediately start getting near to this vibe already. If you then use mid/side processing to keep any lower end mono and restrict an reverb stereo width, this also helps. But then, once you add subtle saturation, maybe add a sampled layer of genuine tape hiss and pitch modulation ( to simulate wow and flutter ) you can get very very close indeed to a tape style recording. Maybe it's psychological but I am aware that I have a digital recording TRYING to sound 'aged' rather than a genuine 'aged' recording.
What I can't figure out is whether these type of production techniques are equal to doing the real thing, ie: lets say I buy a Tascam 4 track off ebay, set it up and record some stuff. Is it necessarily going to satisfy what I am looking for, or am I already going to get what I need by continuing trying various emulation methods and production tricks?
Shame I cannot find any sounds online where I can hear a live synth line and then the same synth line after it has been recorded to tape. ( I'm not talking about reel to reel stuff btw, specifically, cheaper cassette tape. )