Some of you may not be old enough to remember ever seeing a D6C!
Have three of these and learned NOT to record with metal tape as distortion was way higher. Genrally, portables (and most home decks) have too little power (many watts) to impress the best sound into Type IV metal formulation. 16 years ago, Sony made the best sounding Type 2 (chrome) tape formulation for these decks that tracked perfectly with Dolby C.
Of course it helped that Sony SET THE BIAS AND DOLBY NR EXACTLY RIGHT FOR THEIR TAPE FORMULATION!
Nowadays, no clue what tape might be optimum. Really loved the mellow sound of TDK Type I ferric formulation with D6C, no sparkle, just great acoustic quality. Second choice was TDK SA-X.
I really doubt any of these are being made today, so even if the deck is in prime condition, finding a good to great tape formulation match makes D6C less practical for quality recording.
Have some D6C recordings on my site from mid to late 80's that matched anything I've done with DAT.
In the Live Performance section suggest 'compared to you' song, Gene Harris, Andy Narrell, rock-out rendition of Radar Love by Code 3, and a few other recordings posted there that state the D6C recorder with/without external preamplifier in the notes.
I mostly used Dolby C for everything (lower noise, most bass handling), but learned to use the EXACT same deck for playback if Dolby C was to track. That's why I still have all three decks to play back the masters they originally made!
Go to:
www.sonicstudios.com/mp3_2slp.htm and realize what an analog cassette deck could do with the right conditions.