The SR-60, SR-80, and SR-225 all sound identical to me. The differences are these:
1.) Grado claims a tighter left/right channel balance on their more expensive headphones.
2.) The type of pads used
a.) The SR-60 and SR-80 use the Comfy pads which filter out some of the treble and allow more 90-100 Hz bass tones to be emphasized.
b.) The SR-225 and up typically use the bowl pads giving a more clean feel to the midrange and making them sound brighter.
There are some others that use the Jumbo pads which I personally have not listened to, but they look much more comfortable than either the Bowl or Comfy pads.
The thing with Grado is that according to my ear and the measurements they have identical drivers. See the charts below for examples:
In fact according to these charts and many of our ears the Grado headphones with the Comfy pads sound a little more fun because they give the bass a much needed emphasis. Otherwise most Grado headphones are quite bright making listening to trumpets and other instruments quite piercing.
Now if you are just getting into hi-fi and listen to a ton of rock and metal the Grado SR-60 or SR-80 if you want guaranteed channel balance, are great options. They have the same drivers as the other high end Grado headphones, but cost much much less. Many of us have owned a Grado or two. I have owned everything from the SR-60 to the RS-1i. Most of my draw to them was my initial experience going from say an ear bud to a Grado. The improvement was significant and owning the retro-looking Grado is pretty fun when one totes a walkman around. Times have changed and there are myriad headphones out there that sound just as good, but are way more comfortable, and cost less.
Headphones to consider:
1.) Grado SR-60i
2.) Creative Aurvana Live!
3.) Audio Technica ATH-M50
4.) Sennheiser HD-449
There are many others to choose from and I am certain people will chime in here ( Mal in particular ). In the end I would have to shy you away from Grado headphones unless you only listen to rock and metal and can stand the slight discomfort of a hard surfaced headphone resting on your outer ear for hours. The SR-60i is not the most painful Grado to wear and when the fit is adjusted one can suitably wear them for hours without having too many issues with comfort.
Good luck on your search for the perfect headphone.