So is there absolutely no reason not to get the Amazon $269.00 for the HD 580s price rather than the $399.95 for the HD 600s Amazon Price?
PS for a moment there I thought I was finding a different frequency curve for the HD 580 as compared with the HD 600 but now I'm having a hard time trying to find any frequency curve for the HD 580. Does anybody know what the story is?
The HD 580 is a legacy headphone that the HD 600 evolved from. You will not find a new pair of HD 580's.
You should be able to purchase the HD 600 for close to $300 if you shop around.
EQ does not "change" the soundstage of a headphone. Subtle changes in the balance of the various frequencies can create the illusion of different imaging. For example, if you increase the bass, it might make the midrange sound a little "muddy" and this could give the impression of a slower and more closed in sound quality. Increasing the treble can create the illusion of more "air" and distance between vocals and/or instruments.
Regarding your earlier question about angled drivers - they are supposed to move the soundstage more in front of your head to better replicate a speaker type sound, but I have never really noticed this effect.
At the risk of confusing matters further for you - and don't take this personally, but at your age you no doubt have a mild to moderate hearing loss. I have a moderate loss myself and this means that anything anyone else tells you about the sound of a headphone will probably not be applicable to you.
With that said, I think you have been steered down the wrong path by choosing a tube amp and $300+ HD 600's. Tube amps and tube rolling can be nostalgic and fun for some, but you can get the same sound with a solid state amp and EQ. I have owned the HD 558, 598, 600, 650, and 800 and in my humble opinion, I think the HD 558 or HD 598 are a much better match to your listening. Plugged directly into your music source and EQ'ed to your taste, they will give you a huge leap in fidelity compared to your current setup. The 500 Series Sennheisers have a more open and airy sound quality than the 600 Series, and are much easier to drive than the 300 ohm Sennheisers so no separate amplifier is required.
Save yourself $500 and buy yourself a pair of HD 598's...