"Harmon Kardon tape decks are sooo good ! Hard to believe they are tape when listening to a good one."
That experience is common to any top end cassette machine I would say Mark.
I have had a Denon with Dolby HX and 'B' and 'C'. Later a Sony with B&C and a later, simpler Denon. All these machines WHEN BIASED AND SET UP CORRECTLY were capable of reproducing a CD with virtually no audible loss.
Depends to a degree on the music genre. Rock/pop you will never tell the difference, maybe a very wide dynamic range choral piece, Beethoven's Ninth say might be a bit strained at the finale.
I have tried virtually all the tape types over the years except metal but fixed on TDK AD as an excellent type for most recordings and TDK SA for the very highest quality results. AD is type 1 but needs an unusually high bias. SA is T2 and equally a high bias tape. Maxell, BASF, fUJI, Philips are also very good tapes but it is vital that the machine is biased correctly for them.
Myth buster. Dolby NR does NOT cause a treble loss IF the machine is properly setup and the system used correctly.
Dave.