My PS1000s, which were supposed to be "a bit dark" in Grado circles, sounded sibilant on my Yamaha "Natural Sound" RXV363 receiver. Apart from the audio flaws of the Yamaha, the sound mix coming out of the headphone jack is just sibilant.
Sibilance can be described as an excess of treble. Sound should be balanced between highs and lows. Too much in one area can create a perceived deficit in another. After all, how much treble is too much? How much is enough? It's all relative to what's happening on the other end of the spectrum. I've heard tweeters that sounded like a little much, but not nearly so when the bass was cranked up. I've also heard woofers that sounded muddy - until the treble was brought up. It's not some objective amount that pains or pleases us, though too much is always too much and too little is always too little. We are far more sensitive to the mix.
My GS1000s were sibilant, something I attributed to the G-Cush "jumbo" pads. I got all kinds of advice from people telling me to wash them, flatten them, even freeze them or put them in the dryer. I ended up slicing them back a few millimeters to fix the problem. But later, when buying a second pair, to compare these "modded" jumbos with the originals with their sound off an M^3 with a variable bass boost, I came to realize that the problem was not the pads. When paired with a strong amp, the GS1000 was the only Grado (at the time) to sound open and free when the bass boost was cranked up. It takes a certain airflow to get good bass from Grados. One way to get it is to amp with a strong amp, particularly one with bass boost.
Another way to get there is to vent the driver backs, essentially airing them out so they allow the back wave to freely pass through the holes in the plastic blasket. I've done this on Grados up and down the product line (Grado does this on the GS1000 and PS1000). It really beefs up the bass - but from an open perspective. Vented drivers don't sound bloated or veiled; they do, however, sound full, something you may not have experienced with your SR125s on their own.
I do this mod with a ballpoint pen. Take the back off, find the ring of ten holes that circle the driver. Find four good spots (more if you like) and poke through the soft fabric on the driver back. This will increase the volume of the air chamber, which was largely kept out of the loop when those vents were closed or, at best, semi-permeable. Use a ballpoint pen because the rest of the pen will be too big for the hole, allowing you to stop before you puncture a driver.
I'd bet you even money that your SR125 will send 100% better vented. If you don't believe me, vent them and find out. Of course, while you have the hood up, why not damp the magnet back, eliminate the button and eliminate the plastic rear grill. And if that isn't enough, remove the grill cloth. Sometimes, less is more.
