As a former (long time) owner of an HD-590, and a current owner of an HD-595, I can tell you that the HD-590 is definetly not the closest Senn to Grado sound. In fact, the HD-590 was very far from the Grado sound. The HD-590 exemplifies the famous "laid back mids" of the Senn sound, quite the oposite of just about any Grado. The HD-595 gets closer to the Grado sound, but in my opinion, not close enough. I have never had the honor of hearing an HE90, so I can not comment on that.
If you are looking for Grado like sound with enhanced comfort, your best options are a Grado with Zeta pads from
www.headphile.com, or probably something from the Beyerdynamic line. I am currently using a Beyer DT250/250 to complement my SR-225s. The DT250s have a similar sonic characteristic to the Grados and nice warm mids. The DT250/250 are the closest thing I have found to Grado sound in a closed and/or circumaural can. They will never replace Grados, but they are a very nice alternative in both sound and comfort.
For a taste of the Grado sound, look for a used pair of SR-60s. Try them out and decide if you like the, before spending too much on higher Grado models. Either way you can probably re-sell the SR-60 for all or nearly what you paid for them, and then upgrade to a better Grado, or something else entirely.
