"You can usually hear the flatlined high end from the decks i have heard"
I can readily notice the falloff in frequency response as well....but not everyone has such good hearing, some people may notice the difference, some may not....
"I tried it and found it ok for Q8s"
The frequency response of a GOOD Q8 deck will be more comperable to a GOOD 4 channel cassette than a GOOD reel, that's for sure! A GOOD 4 channel cassette will probably outperform a mediocre Q8 deck. For this application, it should work just fine, but you'll have to end up splitting the recording among two cassettes at the faster tape speed....
"but reels and records- nope"
You probably won't notice any loss of quality other than generation loss if you're dubbing from a mediocre reel deck, but quality loss when dubbing from a GOOD reel deck will most certainly be noticed if your ears can hear those highest frequencies, and they're even on the source tape in the first place....
As far as records, a loss in quality *should* be much less apparent with CD-4 sourced material compared to SQ or QS, due to the 15khz lowpass filtering used in CD-4 demodulators. For an SQ or QS source, you might have a stereo cassette (or reel) deck which offers better frequency response and fidelity than a 4 channel job, or even better, a DAT deck....the decoding would have to be derived upon playback, of course.....
Also, keep in mind that overall fidelity and accuracy can mean just as much if not more than ultimate frequency response....the bottom line is that using a 4 channel cassette deck is a compromise and a tradeoff between the advantages and disadvantages.....record something from the best source that you have, then let your own ears be the judge, your own ears will tell you if the tradeoff is acceptable or not.....
Yours Truly,
john e. bogus