I recently installed a Killer Guitar Components (KGC) brass block in my partscaster, which up to that point was fitted with a Callaham steel block. For background purposes, this guitar consists of a poplar MIM Fender Standard Strat body with 2-1/16" vintage style bridge and bent steel saddles, Raw Vintage springs, 59 roundback Warmoth maple/rosewood neck with SS frets, Gotoh vintage-style tuners, Kinman Blues pickups and quality electronics, including Jensen PIO tone cap; strung with 10-46 Pyramid Nickel Classics and played straight into a handwired tube amp via a 10ft Vovox cable. Appearance & Installation: Both blocks are very well engineered and beautifully finished. They are a breeze to install and they attach to the bridge plate without fuss. KGC uses longer steel screws than Callaham and KGC is slightly heavier: Callaham weighed in at 10-1/8 oz (284 grams), KGC at 10-7/8 oz (309 grams). Other than that, and the fact that one is brass and the other steel, there really isnt much difference in quality. Clarity & Tone: Both are very clear with excellent string to string definition. Callaham is quite bright, especially on the high E and B strings, which could be good if you have a dark sounding guitar and/or amp. KGC sounds more balanced, warmer on the unwound strings yet very focused, and punchy and piano-like on the bass strings. Played acoustically both sound powerful, with maximum vibration transmitted to the guitars neck and body. Callahams unamplified tone is thinner though, while KGCs is fuller and sounds more like youre playing a semi-acoustic than a solidbody. Sustain & Touch Sensitivity: Callaham has great sustain, but KGC is even better and rings out what seems like forever. Touch sensitivity seems equally good with both. Trem Arm Fit: Callaham uses a Delrin insert to stop the arm from feeling loose in the block, resulting in responsive and accurate control of the trem. Callaham recommends using a 10-32 threaded virtual pop-in arm with his blocks. These arms have a shorter thread that only need a couple turns to screw into the block. However, they still swing around freely once inserted, so you cant park it in a particular position. Also, the Callaham arm I have barely clears the tone controls when screwed in, which may or may not be an issue depending on your playing style. KGC uses a slightly different approach: a piece of nylon is pressed into the block approximately half way down on the trem hole which is then tapped for the trem bar, resulting in a positive fit and a trem bar that will stay in place no matter what position you decide to leave it in. The KGC block is tapped for standard 10-32 Fender style trem arm (with normal thread length). Conclusion: For many years I was quite happy with the performance of the Callaham steel block. It is well-made, has great sustain, and the trem bar doesnt wobble like in most blocks. I always felt there was a certain brittleness to the attack of the unwound strings though that I simply couldnt dial out. KGC has a warmer, dare I say more vintage sound without harshness, yet with clarity and punch that equals Callahams. KGCs sustain is out of this world and they have designed a better system to fit the trem bar IMO. Bottom line, the KGC block is staying in my guitar! Disclaimer: This review is based on using both blocks in MY guitar and evaluated with MY ears. Your playing style and what you consider good tone may well be different than mine and I respect that. Im not affiliated with either company.