One thing you should know about your amp is that it has a very VERY high bias which means that it will basically run at full power all the time. This is called "Class A" operation and it signifies that the output transistors are each swinging the entire signal. What this also means is that they can't have much power output and that the amps will run VERY HOT. The DR3 is only rated at 40Wpc which is not much and may not be enough for your speakers and desired volume levels.
You may wonder at this point what their advantage is. Well, Class A operation is considered to be the cats meow of amplifier architecture, imparting a warmth and liquidity to the sound that you can't get with more efficient designs. More quality, less quantity.
Your amps were very high class in their day costing $5000 in 1984 which was serious chunk of change back then. They are surely very good - you just won't be able to blast the volume level. How large is your listening room? Do you have any specs on your speakers?
The DR3 also came in a "VHC" version which stands for "very high current" which I think put out 45W instead of 40 but can deliver more current (but not more voltage). You can check with Classe to determine if yours is this version or not. If you decide these amps aren't for you, I am sure you will find a buyer on Audiogon.
As for a preamp, I would recommend a Classe CP50. I used to have one and it was extremely good. A nice deal on one today will be around $850. It has a very fancy all metal remote that comes with it.
You can do some searches to learn more about your amp's style. You got a high quality one but one that isn't just a "plug-and-play" kind of thing without knowing more about it first. Good luck! Arthur