Through unfortunate circumstances........

Many years ago as a young sprite, I knew a gentleman in the Air Force who was engaged to mother that was in the Army. He was born in Germany as his father was in the Air Force as well so it was fitting that he ended up back in Germany during his enlistment. While there he was able to assemble a wonderful array of electronics and brought them back to the states at the end of his deployment. He had a Pioneer system that only 2 people could touch... him & I. Talk about loud in proud!! I always said that when I turned on Rush, not only did I enjoy it but so did the rest of the block lol

Fast forward to 2020 and now I own the Pioneer Spec1 & 2 pre/amp and I will enjoy putting a system together starting with these & will enjoy a little of my youth as both he & my mother have gone home. So there in lies the question. With the 1976 technology was, can I put a modern A/V receiver on this without getting a separate DAC? As I said, there's no system yet but I do have the beginning pieces. The A/V receiver I have currently is the Yamaha HTR-5660 that has speaker outputs that I could cable into the tuner jacks of the preamp. Considering digital wasn't a thing in '76 and I may already know the answer, I thought I'd ask anyway.

I have a Pioneer CD (PD-F906) player & equalizer (SG-9800) so I know these will be a part of the system, but need to make sure the CD is compatible. I know it's not the greatest thing but it is my thing nonetheless. Thank you gentlemen

And, given the age of the equipment, I would have it checked out.
The caps on the amplifiers will probably need replacing.
As it turns out Bob,  he was an electronics guy by trade & we had discussed him opening it up & checking the caps. In his passing the last thing on my mind had anything to do with this & wouldn't have asked. So, when I'm ready to set this up, it will be checked just because when I move this 55lb (amp) monster to where I want it's staying there period, thanks God the pre is only 25lbs. 
To use your Yamaha HTR-5660 with your Pioneer Spec 1 preamp, you’d run an interconnect from the main RCA output of your receiver (the main output rca sockets are just above the left hand side of the speaker outputs), to an aux input or tape input on the Spec 1.
Another way to connect it so that you don’t need to use the Yamaha’s volume control, would be to run an interconnect from the Zone 2 output (and then Zone 2 operations on your Yamaha would be used), or the sound output from the VCR 1 or 2 outputs, to the aux or tape inputs. Try it and see which would be the best connection for you.
Don’t connect the speaker terminals of the Yamaha to an auxiliary input of the Spec 1. Apart from being electrically risky, the sound level will be far too high for the Pioneer preamp.
The Pioneer Spec1 and 2 are both excellent high-end units! And still very desirable today by collectors of vintage audio gear, like myself. Keep and use them as the centerpieces of your stereo system.