"Finer points"...
If you use a front end mixer to get optimal levels to the Tascam, that's entirely up to you, but the Tascam should be able to dial up optimal levels from it's normal inputs without an external mixer.
You may multitrack at "Normal" (low) speed, but the only advantage you get is increased run time, while simultaneously diminishing the top frequency response. It's a trade off and okay, but best fidelity is achieved at the High speed.
You may also track on the Tascam to the tape at "normal" speed and pop the tape into an external standard stereo cassette deck, with considerations, but it's not the best way to leverage the Portastudio technology. With Normal speed and Dbx "Off" you may record to the Tascam in standard stereo format, but it won't be the best use of the format or yeld the best results. It's doable and everything bears out a little experimentation until you figure out exactly what you want.
I think you may be missing a finer point of the Portastudio. In general, it's not intended for you to record tracks on a Portastudio and just *pop* the tape into a normal stereo playback deck, (Sony or other). It's expected to get hiss at that level of playback, because the Tascam records with dbx and the Sony decodes with Dolby, which is a total mismatch that won't address the hiss issue properly. You cannot make a valid judgment of sound quality under these conditions. Regarding hiss, pan or anything in between, all bets are off. What you heard in your experimentation is normal, under the circumstances.
What's intended is that you track to 4-tracks on the Tascam, and once your tracks are filled you play back on the Tascam while mixing to stereo on the built-in mixer, applying EQ, Effects, etc., at this stage. Once the mix is manifested on the Tascam, plug the Tascam's Line Outs in stereo to the inputs of your external (Sony or other) recorder and dub your mix in it's entirety from the Tascam to another tape at a normal speed, track format & NR for standard stereo playback in a standard consumer deck, car deck, boom box, etc. The Tascam is meant to record and playback with dbx NR "On" at all times, (most optimally at High speed) and as such will reduce hiss capably as well as boost overall dynamic range,... i.e., it will just sound better & you'll be closer to getting optimal results from the Portastudio format with forethought and proper use.
However, in one way or another these points are covered in the User's Manual.
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