Well, it'd be correct to say "Grado's SR60s" in that sense, millerdog, but it would also be correct to say "Grado SR60s," since that's the complete make and model. Just like you can say "Ford's Mustang," but in most circumstances it works better to say "Ford Mustang."
It's true that people greatly misuse the apostrophe. In most normal grammatical situations, apostrophes should never be used to pluralize anything. Certainly writing "Grado's" to mean multiple pairs of headphones is wrong. But here at Head-Fi and other places where abbreviations and acronyms and model names (often that don't mean anything) are commonly discussed, the rules blur.
Take for instance, an abbreviation such as that for the baseball term of runs-batted-in, RBI. Often it will appear as "RBIs" or "RBI's", but both are technically incorrect because the abbreviation is inherently plural. This applies even to abbreviations that are not commonly plural -- take POS, the oft-used abbreviation for that pleasant visualization, piece of ****. If I were to write "Senn HD600s are POSs," I would be wrong, because the plural within the abbreviation is pieces. The abbreviation for pieces (plural!) of **** is still POS. It's kind of like when you write attorney-general and attorneys-general, not attorney-generals, except not really.
So anyway, I don't care when people pluralize abbreviations, even though they're wrong. Many dictionaries and grammar texts present rules for pluralizing abbreviations, oblivious, I guess, to the logic of language. It really would be easier to just kick someone every time he writes a plural abbreviation, because trying to formulate rules for the process is, well, difficult. Take, for example, our friend POS. In speech, if we were to pluralize it, we would say it with an "es" sound on the end. So should we write POSes? Clearly not, since we only pluralize things with an "es" when they end in an "e" (and a few other rare circumstances).
So now we're just more confused, aren't we? I think Webster's and some other literary ruler-makers actually suggest (agah!) using an apostrophe to pluralize abbreviations and acronyms ending in "s." (Did you see that? Of course I didn't mean ending in an "s" and a period, but that's what it looked like because I crammed both inside the quotes. The rule tells us that you can't have punctuation following a quote -- but I don't want it inside the quote either! What to do!) Similar confusion occurs with acronyms, those damn cute abbreviations that spell out words. If, for instance I had the acronym CACTUS, which stood for, say, Citizens Against Corny Teutonic Uber-Soldiers or something similarly strange, how should I pluralize it, given the unimaginable need to do so? I could do the standard CACTUSs or CACTUS's, or going by speech, CACTUSes. But then recall that the correct plural of cactus is cacti. Hmm.
Also, what about the differences between American and Commonwealth English? For instance, we Americans write our acronyms in all caps, but Brits and their crew often write them with only the first letter capitalized. See AIDS versus Aids. Obviously you use whichever version you're supposed to use (if you're an American in London, I don't know what you do), but the difference between the two will also lead to differences in pluralizations. Let's adhere to our rules and we'll just let the Brits be wrong.
Numbers seem a bit more clear -- there should be any reason to use an appostrophe, should there? 1990s works, so why use 1990's? Well, I don't know. I use the former, but some people, for instance Funk & Wagnalls, if I recall correctly, say the latter is correct, at least for years.
But back to more relevent topics -- model names/numbers. Whenever I write Sennheiser HD600 as plural (which I do rarely, since I don't own even one pair), I write HD600s. But I can defintely see how using an apostrophe would be acceptable, if not better, since in my version of the plural it may appear that the "s" is part of the model name/number, when in fact it is not. (Of course, I don't think that's the logic most people use for pluralizing it HD600's, but whatever.) Then take the Etymonics Research ER4-series. Two varieties, right? -- the ER4s and the ER4p. Well now. Notice in the first that we already have a lower-case "s" at the end. So it probably wouldn't work well to write ER4ss -- there I can definitely see how writing ER4s's would be better. Then again, we say it with an "es."
A solution to all these kinds of problems, though tedious and somewhat odd looking, is to italicize the model name/number and not italicize the "s" or whatever you use to pluralize it. Very subtle, elegant approach, if you ask me. But if anyone ever did that, I'd shoot them for being anal-retentive. I don't like anal-retentive people.
Anyway, my fingers hurt. Next time you want to pluralize it, do whatever the hell you want. I don't care. I'm sure we'll all understand you, even if some of us will also be sitting at our computers muttering, "****ing idiot. Learn to write."
kerelybonto