I like the Grados, but if you are looking for comfort and especially off-the-ears comfort they are not it.
MP3 files are lossy-compressed. You can't get the data that has been removed back, so converting them to another format will only make them sound the same or worse. MP3s are not necessarily bad; depends on how heavily compressed they are, and the music itself. I'd listen to what you have and see how it sounds.
I believe that the comfort level of Grados is sufficient with the plastic variety. The SR80i I suggested is so light on your ears that if you complain about discomfort you're a sissy. The bowls hurt only on the heavier Grados, the wooden and worse yet the aluminum (like my SR325i) are VERY uncomfortable with the bowls. For my 325is I use the Sennheiser HD414 pads, which offer a warmer sound and a greater level of comfort.
Also, Grados have a interesting headband design. The headband is actually a thin metal strip, but a sturdy one. This means you can pretty much custom tailor the fit of your grado headphones to your head by bending it; eg I have a bit of a cone head, so I bend two divits on the left and right of the center and this gives me a tight by very comfortable and reassuring fit. Now, the SR80i doesn't have the leather sheathe but this is easily fixable if you find that you want it, jmoney leather hand bands tend to be affordable.
Remember how I said I use the Sennhiser HD414 pads? Well there is a MYRIAD of pads available for grado headphones and they're all compatible. S (comfy), M (bowl), G (bagel), HD414 (sennheiser pad, about 5$ a set), etc. And there are more options for people who want to go deeper. Basically, if I had to say so, the single most customizable headphone on the market are any given grado product. Some cans respond better to some pads, and I wouldn't reccomend bagel pads on the SR80i for example, but bowls, any day of the week.
I've been amazed at the difference a USB DAC makes even listening to 192k streams on Pandora (I bought the 1 year PandoraOne membership) and I'm just using a FIIO E7 I got for $75 shipped. Because of this, I can't imagine buying a headphone amp that doesn't have USB DAC functionality + portability if you're mobile. Once you have that, then I'd consider a stationary headphone amp. Thankfully it's a hot marketplace for headamps so there are many to choose from and the prices are coming down on many.
The Bit-head I recommended offers all of this functionality. The batteries are the compromise here, unfortunately in that they are not recharged by the device. They use standard consumable batteries. You have easy access to them so it's pretty easy to swap batteries out. Whether or not the performance is acceptable with any type of recharge-able battery I don't know, never tried. Rechargeable batteries offer less current than non rechargeable, thus the compromise.