This Wiki page will provide links and other pertinent info for all the Grado/Alessandro mods discussed in the SR60-Mod, the SR60 Mod II and the Post Your Grado Mods threads.
The Grado/Alessandro prestige series headphones are open, entry to mid-level headphones popular especially for Rock music, but also for genres such as Jazz and Acoustic. They are known for their sparkle and lifelike highs, but are often noted as lacking in bass. Their real beauty is in the modularity of their design and the ease with which they can be tweaked and modded.
Listed below are the most popular mods for the Grado/Alessandro headphones. Along the way you will notice that it is almost possible to build an entirely new headphone from scratch, using only the flat metal headband strip that originally came with your stock Grados.
Pads are one of the quickest and easiest ways to change the sound on your Grados. There are several types out there:
  1. G-Cush: these are the pads that come on the GS1000, PS1000 and GS2000. They tend to increase soundstage, but weaken the bass. 
  2. L-Cush: these come on all SR series headphones from the SR225 up, all the RS series headphones and the PS500. These are the most commonly used. They are an effective compromise between the soundstage you get with the G-Cush and the bass you get with the S-Cush. Some have gotten improved bass response on the L-Cush by covering their outside circumference with electrical tape. 
  3. S-Cush: these are the stock pads that come on the SR80, SR60 and the SR125. They bring up the bass nicely but also impart a warmer sound signature until you "quarter mod" them by cutting a disc of material out of their center, rendering them similar in effect to the TTVJ Flats.
  4. Sennheiser HD414 pads: Aside from being a brilliant shade of yellow, these are a nice substitute for the S-Cush if they can be had for less money. They can also be Quarter-modded.
  5. TTVJ Flats: these are made specifically by TTVJ. They are intended as replacement parts for those still using the much revered HP series headphones by Joseph Grado. Used on other Grados though, they have the effect of bringing up the bass nicely without adversely affecting the highs and mids. 
  6. TTVJ Deluxe Flats: these are also made specifically by TTVJ. They are a refinement of the original TTVJ Flat that appears the same, but is stated to give tighter bass and more refined highs.
Driver Modification:
These two modifications involve altering the drivers. To get at the drivers is fairly simple and easily reversible because the only glue holding the front and pack half of the cups together in the SR series is hot glue. A tutorial on opening the cups is found here.
  1. "Hole Punch" mod: generally only suggested if you have plastic cups and you are looking for more low end response. Open up your Grados as per the tutorial referenced above. Look at the back of the driver with the ear-side (with the white gauze on it) facing a light source. Note the slightly brighter spots that indicate holes in the plastic underneath the felt that you see around the magnet. Using a pen, or similar blunt, but pointed, object, pierce the felt ring over the holes. You should only punch hole one hole, re-assemble your headphones and test them out for a few days to be sure you are comfortable with the change in sound signature. If you wish to reverse the hole punch mods, muslin is a good cloth of choice to cover up the holes. It is much easier to poke holes than cover them up so take your time and make sure you want to punch the holes. 
  2. Blu-tack/Dynamat/Sorbothane: placing a small pea sized amount of dynamat on the back of the drivers, avoiding covering the vent holes, is suggested to try and tame some of the resonances in the metal plate covering the back of the driver. Grado actually does this with several of their higher end models. Dynamat is typically the preferred material, but favorable results have been gotten with Blu-tack and Sorbothane.
Alternative Drivers:
For those of you who have taken a Grado driver as far as you can, there are several venors who sell aftermarket drivers you can install as well. There are currently three people with branded drivers of their own: Nhoord Audio, Elleven Acoustica and Symphones. A simple Google will take you to their websites. I have linked threads devoted to each in their names. Some have also liberated and used the drivers from the Sennheiser PX100ii with great effect (especially with full wooden cups and G-Cush pads). The SennGrado owes its invention to fellow member @wje. There is a thread devoted to it here. Others have made closed-back Grado-style headphones using the Koss KSC-75 drivers. There is a thread devoted to it here. This one owes its design to fellow member @stratocaster
Some modders claim to see improvement with custom cables, especially silver. Regardless, replacing the long, unwieldy, hard plastic stock cable is a must for reasons of comfort at least. Something sleeved in Paracord and terminated at a reasonable length of four or five feet makes using and handling your Grados a little easier, especially if you require more portability.
Cups/Cup Structure:
Those who own the SR series will find it beneficial to upgrade their cups from the stock plastic to wood. There are two types of cups:
  1. Slip-ons: these are designed to replace the outer plastic shell (not the plastic inner sleeve). This is a simple mod to perform. Follow the tutorial for opening up your headphones, replace the plastic outer cup with your wooden ones, close everything up again and enjoy. This mod does not give the sonic upgrades associated with a fully liberated driver in a completely wooden cup, but it is aesthetically pleasing, and alters tone very slightly.
  2. Full Cups: these require full liberation of the driver from its plastic housing. You can see a tutorial on how to do this here. Fast forward to the 1:15 minute mark. The driver, fully liberated from the plastic sleeve, sits in a single piece cup made with wood. 
Its worth talking a little more about wooden cups here, specifically ones designed for a fully liberated driver. Key things to consider are:
  1. Wood type used: denser woods tend to produce a more "neutral" and cold sound. They are extremely durable though, and more exotic looking. They tend to be more expensive as well. Softer woods are "warmer" in nature. Favorites among members are Walnut and Mahogany. More can be found on tonewoods here.
  2. Driver mounting method: some cups are made to accept a driver with a strip of adhesive foam wrapped around its perimeter. The resulting sound is usually less influenced by the tone of the wood used in the cup and more influenced by the driver itself. Conversely, some cups are made with a "press-fit" for the driver. No foam tape is needed, the driver seat is tight enough to take the driver alone, resulting in direct contact between the driver edges and the wooden cups. 
  3. Cup Shape: generally more mass is known to give a bigger bass presence. Having thinner walls can change tone, as well as the design of the area the pad will hook onto. Some have made adapted pad lips that hold the larger, more comfortable Beyerdynamic pads designed for the DT770, DT880 and the DT990. A good thread on cup design documenting @thelostMIDrange's journey through finding the best cup for his tastes can be found here. He also goes into finishes, pad lip design and wood types.
  1. Aluminum: some have great success using a pair of SR325 headphones as their base model, and replacing the plastic inner sleeves with wooden ones (to mimic the design of the HF2 and the PS500). The aluminum encasing the wooden sleeves is said to impart the tonality of wood, but in a more restrained fashion, rending the effect of a more "polite" and refined sound. A few third parties make and sell aluminum cups. Others have sourced aluminum inner sleeves and replaced the stock plastic ones with them to great effect as well (@ambchang has extensive experience with this). A few have had full aluminum, one-piece cups milled as well. The primary issue with full metal cups is the potential for ringing. This is fixed by lining them with various materials, like felt, or Dynamat.
The stock vinyl headband pad on the SR series isn't very impressive. Several people make leather replacements you can install yourself. Some mimic the stock leather headbands used on the RS, GS and PS series headphones, some improve upon the design with better padding and better quality leather. This is, for the most part, an aesthetic upgrade and does not affect sound quality, only comfort.
Another option is aluminum gimbals and rod-blocks, to replace the stock plastic ones. This is only for aesthetic reasons as well. Again, there are third parties who sell these online, both independently and on eBay.
Some opt to forego the Grado headband assembly altogether and use the headband from the Sony MDR7506, the Sony MDR7502 (if you can have your cups made a little smaller to accommodate the smaller size) or any other headband they find that fits. 
Grado Headphones Overview:
The Grado series headphones are very similar in many respects. All use a similar design. The drivers actually look identical in some cases, but small differences set the various models apart. Here is a frequency response graph of the 4 plastic-body prestige headphones:
It's clear they are all related, but not exactly the same.
The different revisions:
  1. The "e" classification: the "e" series was released as a replacement to the "i" series. The sound chambers in the cups all remain the same size, but the drivers have all been upgraded. Its worth noting that all Grados now come with holes punched in the felt. The RS1i was also upgraded to a 50mm driver. Early production runs had the driver protruding from the wooden cup slightly, bringing it closer to the ear. Complaints regarding comfort were numerous. Later productions runs are now reportly shipping with the driver flush with the cup lip again. The RS2e is also being noted as sounding markedly better than its predecessor, the RS2i. The GS2000e was released roughly around the same time. Grado's new flagship, it boasts Maple-bodied cups, with Mahogany end caps, shaped like the GS1000
  2. The "i" classification: the 'i" Grado's are similar to their predecessors. The notable difference is the larger sound-chamber and the larger diameter of the outside face of the cup, giving it a "mushroom" look. The SR-80 used to come with L-Cush pads, as used on higher models, but the new SR-80i uses the same S-Cush "comfies" as the SR-60 & SR-60i. Also, It has been mentioned that the soldering pads on the drivers of the SR-60i's seem to be less prone to detaching during soldering for re-cables compared to the SR-60's.
  3. The non-"i" and non-"e" original models: these were the original models released by John Grado. They are notably simpler with thinner cables and lower-profile cups. Some of the oldest examples of these show variances between production runs occasionally. Examples of this include subtle differences between the vinyl headband pads, different cable sheathing hardnesses, very rare examples of SR80 headphones with 1/4 inch instead of 1/8th inch plugs and even rarer examples of SR80 headphones made with metal mesh instead of the plastic used on the outside face of the cup. Certain older SR series headphones have drivers whose white cloth over the ear-side of the driver has discolored to a shade of pink. These have been noted to sound slightly different to stock Grados, exhibiting a slightly less harsh and forward sound signature.
A more detailed history can be found here, courtesy of @devouringone3. There is also a fascinating interview with John Grado here, courtesy of @Zanth
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