+3 consistently, and occasionally flashing +6 is about right,...
or maybe a little hot,... depending on the tape.
Keeping 0VU lit solidly, with heavy activity of the +3, but short of +6 might be best. Like I said, it depends a lot on the tape.
Newer tape formulations can handle slightly hotter signals. F/I, a TDK SA/90 may saturate at +3, while a TDK CD-Power/90 probably saturates at +6.
Anyway, you're right, rule of thumb is: print to tape as hot as you can get away with, short of distortion.
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Re: Trim & Faders, when setting input levels,...
-the faders should be about in the 7-8 range, just to start, 'cause that's their sweet spot, and it provides a lot of range of adjustment, mostly down.
-the TRIM pot controls the basic level to the very first stage of the channel, the input/preamp section. It's important to use TRIM to control levels, and minimize distortion right at the input stage, the very first stage of the mixer's signal chain, before any other stage.
-That is, if you're getting distortion of the signal, in the phones, BEFORE printing to tape, then adjust down the TRIM, and/or lower the output signal of the instrument, or signal source.
-If your basic input signal sounds clean, in the phones, but your recorded/playback signal sounds distorted, then you minimize distortion on tape by lowering the record levels with the FADERS, ['cause your TRIM is likely set ok, if you don't hear distortion on the input side].
-The 'warming up' of the signal with TRIM, as G/H said, is achievable, but it's done by overdriving the input section a little, and therefore, inducing a subtle overdriven distortion into your signal. This technique may be used in modest amounts, but taken to extremes, it's bad for the mixer's inputs. F/I, you may 'warm up' any sound by overdriving the Portastudio's inputs slightly, but if you're looking for that 'grungy' guitar sound, you don't want to try to develop this kind of distortion on the inputs of the mixer. You're better off by developing that 'grungy' distortion in a stomp box, pedal, or other external preamp, and then drive the Portastudio's inputs with a nominal, safe signal level.
-Not that you'd do it, and it's a little off topic, but I've read posts about how someone who tried to develop too "fat" of a sound, right in the Portastudio's mixer channel, that they blew out their inputs. -Hey, you & I might not do that, but someone's done that, at least once.
-It's a matter of splitting the difference, and differentiating between whether the distortion is developed by overdriving the input section, or by overdriving the tape, which would be two different things, with two different approaches, basically boiling down to one thing,... If there's distortion, find where it is in the signal chain and minimize it,... turn it down.
