OK. Thank goodness for insomnia - 5:00 a.m. for me.
I did a bit of "informal" G-Cush testing. My source was my Audio-GD "Fun" with the Earth module. Input was via the Optical cable. Foobar was the player. No EQ settings applied. Music: David Bromberg "Use Me" which has some good guitar, bass, as well as drums / percussion.
* PS-500 / G-Cush pads: This is the normal configuration for how I run my PS-500s. The frequencies are all represented with the G-Cush pads. No real loss of bass, good mids, good highs - but not excessive. As to the the bass, some people with the PS-500 and the L-Cush pads have indicated that the bass is quite present. I, too, will agree that with the L-Cush pads, the PS-500 really has a good amount of bass and a bit more than my RS1i has for sure.
* MS1i / G-Cush Pads: This is the first time I've heard the G-Cush pads on a model other than the GS-1000 or my PS-500. I don't feel that the highs were overextended in a fatiguing way at all. The mids were good. The bass was less than my PS-500 offers. But, do keep in mind, the PS-500 is a headphone with more bass out of the gate, and also costs nearly $500 more.
As to the difference in bass between the MS1i and the PS-500, the biggest variable could be the vent holes that are apparently in the PS-500 Grados. I'm hesitant on opening up the holes a bit on the MS1i, as I normally keep it stock as a reference point. It also doesn't get a lot of use, due to just being available for comparison against the other Grados. Brooko is right. In considering hole / vent mods or vent opening, just don't go too crazy. It's kind of like medicine. People think that if a little work "good" then 3 or 4 times more would be "great" - but, that's not always the case and with medicine, it can be quite risky. I'd say consider 2, but don't go higher than three at the most.
Finally, I was able to search a bit and did find the thread where I did initially read about the PS-500, PS-1000 and GS-1000 having their vent holes opened. Apparently these models have "3" holes opened up as suspected in the thread. There is also a mention of the Magnum, version 3 having a few holes opened, too. If I recall, the version 3 may have only been a driver that would have come along with the Magnum upgrade when performed by Rydon. It could easily have been versions at some point after that, which became available for us to purchase just the drivers without any upgrade options.
Here is the thread with the reference to vents opened on Factory Grado models: http://www.head-fi.org/t/576717/grado-modders-go-magnum/2640#post_9507046
I may send a note to Grado and inquire about this since I did own the GS-1000 and currently own the PS-500, I am quite curious.
I think I'll get a few more hours of sleep before waking up to head to work. Enjoy!