I'm going to put my opinion on the subject this way:
I owned the GL w/ DPS, GS-1, and GS-X simultaneously for a short while back in 2007. I formally compared the amps against each other and found relatively minor sonic differences between them. The equipment I was using back then was an Arcam FMJ CD33 (CD player) and my headphones included the AKG K701 (I know I had more headphones as well, but I can't remember which others were used, if any - though I know for sure that I did use the K701). I found more sonic difference between the GL w/ DPS and GS-1, than between the GS-1 and GS-X.
My opinion of the GL w/ DPS vs GS-1 was that the GS-1 "evened" out the slightly-wonky sound of the GL, as I thought the GL had a slight treble emphasis along with a slightly forward upper mid-range. The GS-1 also had what I could only call "clearer" bass than the GL and a slight added touch to the mid-range. It had an overall more relaxed signature than the GL, which had a "tense" quality.
Compared to the GS-1, the GS-X was even clearer-sounding throughout the spectrum and rendered an absolutely, completely noise-free, "black" background. It also completely flattened out that "slight added touch to the mid-range" and to my ears it was the single most invisible, transparent amp I'd heard yet. I never got the impression I was listening to an amp with the GS-X - I always thought I was listening directly to the source.
Headphones shouldn't be an issue unless they're incapable of reproducing a high level of detail - and in my experience, most of the current in-production Grados aren't really detail freaks (though I haven't heard them all - the only models I've heard are the SR60, SR80, SR225, SR325i, RS-1, GS1000, and PS1000, plus the discontinued HF1 and HF2). I haven't heard the PS500, which you seem to own. But if the PS500 is anything like the HF2, then this is just a guess, but I'd guess that it might sonically pair well with the GL, especially if you listen to rock music. (On a side note, I wouldn't personally call the PS500 high-end. Actually I probably wouldn't call any of the current-production Grados high-end. The only Grados I'd call high-end are the discontinued PS-1 and HP1000).
I don't currently own any Grados for an up-to-date GL vs GS-X comparison, though I have compared the amps on my current CD player with other headphones (the HD800 specifically), and my opinion today remains the same as it did in 2007.
My recommendation is that if you don't need the features of the GS-X, or the GS-1, then the GL (or any other Dynalo build) should be just fine for you.
Here are a variety of old threads that I wrote in the past that cover the amps in question and might provide further info:
- Gilmore Lite DPS vs Elpac review: http://www.head-fi.org/t/225178/comparison-review-gilmore-lite-dps-vs-elpac
- Multi-way review of 6 solid-state amps: http://www.head-fi.org/t/278691/six-way-review-square-wave-gs-1-canamp-ec-ss-gilmore-lite-black-cube-linear
- GS-X pics and arrival story: http://www.head-fi.org/t/231415/the-headamp-gs-x-has-landed
- GS-X impressions: http://www.head-fi.org/t/237928/impressions-headamp-gs-x
- GS-X review: http://www.head-fi.org/t/247854/review-headamp-gs-x-single-ended
I'm going to add another recommendation: if you don't have a particular interest in a Dynalo-based amp, or solid-state for that matter, there might be other amps better-suited for the Grado headphones you have or are considering. There have been some tube amps reported to synergize well with Grados, including the Mapletree and Eddie Current amps.
Btw, Grado headphones that I've heard with the GL and I thought sounded good with it: SR60, SR80, SR225, HF-1, HF-2, and RS-1 w/ flat pads (w/ and w/o buttons).