Why are you looking to add a preamp to a receiver? A receiver is a preamp and amp built into one box. Do you mean a phono preamp to allow you to play records? What is your goal.

A preamp controls volume and switches between input sources (FM, CD, VCR, etc.) Adding a preamp to a receiver wouldn't really add any functionality to it, so it would be a waste of money. IF that unit has preamp outputs you could add an amp, which powers the speakers and using the Sony as a preamp. That is the exact opposite to what you were thinking. I don't mean to be cruel but that would be a waste of money too. Sony receivers of that vintage, even the ES units were OK but nothing special. Think basic 1979 Ford Mustang. I'd recommend using it as it is OR replacing it. It really isn't the kind of piece that you'd build a system around today.

To me when building a system it always makes sense start with the speakers that will be used long term because that will influence what kind of amplification would be appropriate. Out of all of the components in a system the speakers have the most influence on "the sound" on a system.

The average person (think regular no-audiophiles) keep at least speakers 12-15 years which is longer than a car. So I encourage them to think of it as a long tern investment in their happiness. For example a pair of $2,000 speakers sound expensive on first blush but when one thinks of them as $134 a year they seem very, very reasonable. I'm not advocating a specific budget for you. Those numbers were purely for illustrative purposes. This all makes perfect sense because modern speakers used normally don't wear out while CD players and even electronics may.