Hey Marbles, Lonewolf and Chair.
Two things I've found not to work; Maggots in cables (which I found out later was supposed to be Magnets) and crickets (which make the same mess maggots do when you try to use them in electrical circuits.)
Equipment mods is probably something I won't peruse with a lot of energy. Ridge Street Audio as of now is mainly about our cabling. That face will change as other audio products from either myself or our associates get introduced and establish the same rep our cabling has. Presently, the Sason Ltd. Granite loudspeaker is creating the same ground swell our cabling has and continues to do. Another product I've developed but haven't formally introduced yet is our Z-der Block; a small AC power feed device that installs between the AC power cord and the piece of audio gear the AC cord would plug into. I've sold a couple and have a couple of these out for audition right now and the response is "Wow!" Further, another associate is still tweaking out the Accure AC device.
But, I do enjoy modding stuff. I don't own a piece of gear I haven't done something to. I've kind of cut my teeth on Adcom, Counterpoint, Marantz, Yamaha, Jolida, Pass and a few others I can't remember. My approach is pretty straight forward; competently engineered stuff that was built to a mid-fi or higher price point. Usually where manufacturers skimp is components quality. Better parts components perform leagues better but are also considerably more expensive. For a manufacturer purchasing a certain component to be used in a circuit, it can be the difference of spending $1000.00 for X quantity or spending $4000.00 for the same quantity. Times that by how many different parts you need to get a functioning circuit and perhaps you can begin to understand why the best Audio Notes, Brinkmanns and Lamms of the world are as expensive as they are. These products are for avid music lovers (many times whether they can afford it or not) rather than hi-fi enthusiasts who like to admire their gear with music and then enjoy their music to whatever degree.
When I approach a piece of gear I usually start at the power supply. Lots of things can usually be done here to get better supply rails for small signal and output device rails. I consider this foundational to any piece of audio gear because the best signal circuitry is only there to modulate what the power supply gives it. A sub-par supply (not uncommon) coupled with excellent signal circuitry will generally waste a lot of performance potential. Then I'll look at the signal circuitry...signal path, feedback loops, etc. Components that I commonly replace are resistors, caps, diodes, inductors, oscillators and ICs. There are some things that can even be done to AC power and output transformers. And, of course, I'll replace and use our wire where it makes sense.
Costs for different projects are based on consultation and are quoted on a case by case basis. I can do minimal stuff or go as nuts as one might want.