Recording a little hot will generally give you the the best results.
Yes, you're right, that you do get a 'better sound', and better S/N ratio when you saturate the tape properly. It's ok to drive the tape as hot as possible, but just short of distortion.
That's for two reasons. First, the newer cassette tapes will hold more magnetism than earlier tapes. Second, that varying on brand, you may actually have to hit the tape harder, to achieve the desired signal levels on playback.
F/I, just yesterday, I tested a TDK SM60 and MAXELL XLII 90 cassette back to back, on my 424mkII, with a 0VU/1Khz text tone.
On the TDK, and I had to hit the tape +3VU hotter than I actually got on playback, so I hit the tape at +3 on record to achieve 0VU on playback.
On the MAXELL, I had to drive the tape even harder than that. I didn't have time to get an exact reading, but I'd estimate having to hit the Maxell XLII tape at +5VU to get 0VU on playback.
This points up two things:
One, is that there's an analog adjustment, internal to the Portastudio that could be adjusted/calibrated to rectify this record/repro level mismatch, to make the record level match the playback level more exactly. [That's a service issue, on my 'things to do' list].
Two, is that tapes vary. Different tape formulations will need to be driven with differing amounts of recording amplitude, to achieve relatively the same playback level. You may benefit by running your own level comparison test. See at what recording level it takes, to get 0VU on playback. Try several brands.
One thing's for sure, is that if you don't get a strong signal onto tape, you'll be boosting all sorts of hiss, when you try to bring in into the mix, on playback.
I record using analog Tascams of all sorts, and I am a huge Portastudio fan, with several Porta's to my name. I've repaired and adjusted reel analog decks and Portastudios, and IMO, most of them are out of calibration.
