Opinions about modifying an older preamp

Any advice about the results of modifying/upgrading an older solid state preamp like Marantz, Harmon Kardon, etc. with modern capacitors, etc.? Who does work like this well, and are the results good? Which parts are most needing of upgrade?
Most anything can be improved by replacing capacitors, diodes, wiring, adding additional circuitry & deleting unnecessary parts, etc... BUT one is always best off starting with something that they consider to be a good product as it is. While one can take a mediocre product and turn it into something that is worthwhile via improved parts quality, you are better off starting on higher ground in terms of solid circuit design as it is a lot shorter climb to the top from there.

You might also want to take into consideration that many older products have little resale value. As such, sinking a good amount of cash into a product of this nature will net you next to nothing should you ever desire to sell it. If you really want to see what the product will do with upgraded components, it is best to try to modify gear of this nature yourself. This will allow you to experience the benefits of improved performance via the modifications and at the same time, keep the cost of doing so relatively reasonable. Sean

PS... I just modified a preamp for my Dad and dropped it off last night. In his own words, "my system has never sounded this good" and i agree. For the record, this preamp was designed in 1974 and had one revision before it was discontinued in the late 1980's. As such, it was "light years" behind in terms of supposed "technological advances". The funny thing is that what i did were very basic mods to what should have been a "hard and sterile" SS design from the 1970's. That is, if you believe what most "bottleheads" will tell you. For the record, the preamp that it replaced was a 3X more expensive tubed unit that had "fancy" caps, a good sized external power supply, etc... from the factory. This is not to mention the upgraded tubes that had been tried along the way.

The reason why this SS "antique" sounds so good now ? Probably because it had more engineering and technology in it back when it was designed 30 years ago than most gear does now. As such, i followed my own suggestion that i stated above i.e. i picked a good circuit and built upon it. Quite honestly, there's still quite a bit more to be pulled out of that circuit but that will have to wait until later : )
Personally, I would compare buying a used high end pre-amp to the cost of buying and upgrading an old unit. You can get something like a Sumo Athena for around $250 and it will blow away almost anything solid state that you are considering.