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

You only need a DAC if you want to stream music/play music from a digital source(computer/NAS) or play music from a CD transport (a CD player has the DAC built in).
It also depends upon how 'high-end' you want to go, as separate DAC's will provide higher quality of sound -though at a higher cost level.
I appreciate the response Bob, I assumed as much. I'm still interested in the DAC & better A/V amp in the future 
Almost always an AVR is so crappy you would be better off just tossing it and using the Pioneers. But they are so old its possible they are not much better than the AVR. Only way to tell is to listen and compare. Keep in mind the quality of the interconnect used between them will have a major impact on this comparison.

Whatever you do, do NOT try this:
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.

Speaker level out put is so much higher voltage than line level that unless your volume is turned down to near nothing you risk frying your pre-amp. Which would truly be a shame as you would then be using your worst sounding component to ruin the one that (probably) is the best.
Thank you Millercarbon, so running a line from the preamp to the A/V reciever is better?
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. 
The Pioneer CD player is fine as-is. It will work well with the Spec1/2. Later you may add a DAC to the digital output of the CD player. 
Don't worry about the capacitor condition of the Spec1/2. In my considerable experience buying/selling vintage gear I have never had a problem with capacitor deterioration/failure in solid state gear. You can get a variac and plug the Spec1/2 into it. Starting at 1 volt slowly ramp up the AC voltage over a 6 hour period. This will allow the power supply caps to gently reform after an extended period of unuse.
Damn, I pressed the wrong button on my computer. DAC 1 PRE is what I wanted to say. It was $500, and on the old side, but it sure sounds great.

Best of luck, regards,
@ roberjerman any particular model you recommend? And don't worry, this is not going anywhere. With your vintage experience you'd likely be a fun conversation

@islandmandan thanks for the recommendation