|I can't believe that Sharp would make the SR60 (which was made around the same time as the 722 and 821) any different by including a better lineout, especially since it was touted as a entry-level model to begin with. I would bet that it's exactly like all the other (newer) models. I would go with the MT90 instead of the SR60 for the reasons you already listed.
I agree with you completely -- the SR60 was the "budget" line, while the MT90 was the mid/high line. The one advantage the SR60 has is a 10mW headphone jack (as opposed to 5mW for the MT90); but if you're going to be using a headphone amp, no biggie.
As for timelines, the SR60 was introduced in early 2000 (and I believe is still being produced), while the 722 was introduced in late 1998. The 821 was introduced in early 1999, just a few months after the 722. There was some overlap in production, but the SR60 is a much newer unit. That said, the 722 and 821 were and are much better units.