The first indication as to whether or not to invest any money in your unit - is it's age. The older the unit, the more likely it is that you DO want to spend money on it. The second indication is it's weight. If it requires both hands to pickup, and you risk a hernia lifting it... Its definitely something you may want to invest in. If your receiver has a dial string tuner, you wont easily replace it's sound with what is available today for less than a grand! If you can pick the unit up with your pinky finger, well, forget it - go buy something new - unless it's a keepsake, has sentimental value or is an heirloom piece.
The silver faced units are almost a given that they are worth investing in. Some older black faced units are also worth your time and money to get them repaired or restored. Just depends on the brand model. You can do a quick search on ebay and see what they are going for - be sure to check the sold or completed listings. With few exceptions, if it's selling 'good used' for around $200 or less - well, it may be time to go shopping for a new receiver or amplifier, as it's gonna cost at least that to get repaired - especially when you figure in the cost of shipping. Most units ship for $40 or less, unless the package weighs over 40lbs and the box is larger than 25" x 25" x 25".
While the older Receivers and Amplifiers may not be THX certified, the sound they put out is much warmer. Comparing these Vintage Receivers and Amplifiers to today's Hi-Tech toys is like comparing apples to oranges - they are in completely different leagues. If you want a home theater experience - go buy the home theater stuff. If you want to listen to some vinyl or you want to rock, or just want that warm sound again, then these Vintage Receivers and Amplifiers are for you. Of course, you CAN have both... One room for theater, the other for listening to vinyl, tapes or CD's.