Pros: Excellent SQ, Value, Lightweight, Modifiable
Cons: Requires some DIYing
Ah yes, the SennGrado. Like a unicorn or a woman with brains and beauty, this one is almost impossible to believe. But I'm here to tell you: believe it.
If you want background info, head over to the non-Grado thread. Basically a handful of modders (led by @wje) have installed the drivers from Senn's portable PX100 II in Grado-style cups with astounding results. If you're not sold yet, consider the fact that most who have tried the SG have considered, if not already sold their higher end Grados.
The SennGrado bests anything in the Prestige series to these ears. They are of the same sound as the RS1/GS1000 with slight differences.
The low end is punchy and dynamic. Nice and full with plenty of texture. I prefer the SG's bass to the RS1i's. It reaches deeper and seems more linear where the RS1 has a little extra in the midbass that seems more round and less detailed in comparison.
In the mids, the RS1 wins as they seem more forward and lush. That's not to say that the SG is a slouch though. still very clear and easy to listen to. For those who've heard both the GS1000 and RS1, the SG's mids are more similar to the GS1000 with respect to how they are framed by the upper and lower ends of the spectrum.
The upper end is nicely extended and detailed. It's not as aggressive as the RS1 or 325 but certainly isn't rolled off either. This treble is what I think of when I think of non-fatiguing.
The intangibles are right on par with the GS1000. Bigger soundstage than the RS or Prestige series. Nice air up top without sounding unnatural.
Overall, the SG is one that any Grado fan should hear once. I've never head a Magnum, so I can't compare, but used PX100s can be had for less than $40, so for sound on par with upper end Grados, a pair of SennGrado's can be assembled for less than the cost of Magnum drivers. Not to mention that they can be installed in whatever cups you'd like so the customization options are endless. Do yourself a favor and check out these exceptional headphones.