This is a question that would have been important in the 80s and early 90's, but I'm afraid you'll find there's not much information (or desire) left to fuel a discussion about this nowadays.

No offense, and I'm not making light of your question, but it's rather moot now....The short explanation is this: Metal Tapes were among the last-gasp of attempts at a higher level of quality back in the late 80's, early 90's, before recordable CDs (and then MP3's) came about it. As "good" as chrome's specs were, a properly aligned metal cassette would be even better. (Please remember that the term "Properly Aligned Cassette" is almost an oxymoron. Any honest tech of the day would tell you: there ain't no such thing.) I remember hearing the differences between chrome and metal tape on well-tweaked machines and was impressed at the time - For what it was worth.

Keep in mind, "Pro" cassettes were pretty much a coined phrase by the manufacturers to sell product. They were perhaps ""Pro" in the sense that a Pro studio would make some client copies directly from the master.
Nakamichi and Tascam made some great products, but fundamentally, they were compromised analog machines, with the usual analog problems and issues. (Azimuth, Cleaning, alignment, oxide shedding, wow & flutter, Hisssssss, etc.)

Open reel was the "Pro" format, no matter what anyone tells you historically.

Few "Pros" would master or put anything important on a cassette. They were largely used for client dubs, temp or proof copies, etc. Yes, folks would attempt to use them for more serious purposes, but in the end, they were never good enough or reliable enough for anything important. So, yes, while "Metal" tape was arguably better, it was not going to keep a soon-to-die-off format afloat. I remember back in those days, that by the time Metal cassettes were out, F1 digital (on VHS & Beta video machines) was making inroads, with DAT hard on its heels. Cassette was dead on its feet for quite a while, long before most folks knew it.

I remember aligning machines for the elevated levels and better Freq response with Metal formulations, but compatibility was always an issue as well: Not everyone had a metal position swtich on their machines, so it was always a worry that we would be making tapes that were incompatible with client's machines. Add to that equation that few consumers REALLY knew (or cared) about the difference between normal and chrome bias settings, ditto for Dobly B, C and HX. Metal was, IMHO, just one more way to confuse people and sell more tapes. Most people just shut that stuff off, always seeking the "Brighter sounding" selection. (Sad but true...)

Finding someone nowadays to explain why it's simply not being pursued is going to be tough, I would imagine. Why is vinyl gone? 78s? 45s? Every medium has it's day, and the same with cassettes. Metal Cassettes were just too little, too late. (Same with the ill-fated DCCs)

Does it (will it) help you get a better, warmer sound for your bounces? Maybe.....certainly worth the try. I've done effects-loop through 3-head cassette machines in the past with OK results. It's not as good as doing the same thing with an open reel analog tape machine, but it's fun to try it. The difference you'll get between metal and chrome may or may not be worth your time fretting over it.

The specs you mention (listed on the packaging or wherever) are also something of a cruel joke. You MIGHT get those specs if all the planets are in alignment, the wind is blowing in the right direction, and its' a leap year. (Ok, maybe only Feb. 27th.) Trust me, there's no cassette out there that's REALLY going to let you hear any difference between 20K and 21K. (Hell, most folks can't hear that high anymore, anyway.) Most times, the Metal WILL sound better (slightly) than the Chrome tape, but for what your'e doing, I'm willing to bet you the major difference is simply alignment issues themselves.

Good luck, however you go. (Does anyone actually SELL metal tape anymore? Seriously, I haven't bought cassettes in about 5-6 yrs now..)

Thanx a lot, Joe!
I forgot to call my question a "retro-question")
Messing around with tape types is not so important as I see. So... I have an old 1/4" open reel in the basement, but even don't know the manufacturer) Going underground...
Is open reel's wide frequency range more 'real thing'?

Is open reel's wide frequency range more 'real thing'?

Absolutely! Yessir. 8-)

I've heard from many on these forums and others that even with digital equipment, they mix down 1/4" stereo open reel recorders, and I've tried it to great results in my analg/digital studio (I use digital tape recorders and the rest of the equipment including the mixing console is analog). Always works for me, you should do some searching on the forum for info on it!