Falling out of love with tubes

" By “going direct” I meant plugging the headphone out jack of my computer into the input of the SA-250 amp and using the volume control of the computer. Also, my Denon cassette player has an output level control. Thanks!"

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A few things, your tube amp is a very old design, modern tube amps are a lot more neutral and detailed. To go from your PC to the amp, like you’re doing now is pretty bad, you’re losing a lot of detail, both from the PC volume control, which is digital as well as the headphone out, which isn’t the best quality, it’s basically an internal headamp board based on cheap components and possibly not a lot of gain. Look for a cheapish external DAC/Preamp, with an USB input, then you can hook up your PC to the DAC/Pre via USB which will transmit the audio signal digitally, without loss...then the DAC/Pre will convert the digital to analog, amplify to line level, and send it to your Conrad Johnson. You will hear what a difference that makes.

Few examples:
https://www.crutchfield.com/p_772D1/Audioengine-D1.html?tp=59340

https://www.crutchfield.com/p_779DACMPLB/Cambridge-Audio-DacMagic-Plus-Black.html?tp=59309

Boy, the Eico was kind of a budget amp even when it was new- ceramic caps for coupling caps, cheap parts throughout... that would be a good amp to fall out of love with, and I would not abandon tubes on its account!

Tube technology has improved since the ST-70 (which was designed in the late 1950s); there are better filter caps, coupling caps, resistors, controls and there are better circuits. Even if your amp was refurbished it could easily be outclassed, even by other refurbished vintage amps like the Dynaco ST-70 or the Harmon Kardon Citation 2. But there are modern amps as you have learned that bring a lot to the table- its a big world.

Running out of the headphone jack of your computer is not the best way to get good sound out of it. IMO/IME you are better off with a DAC that runs off of the USB port and the appropriate software to make that happen (for example JRiver if you are running a music server). You very well could make good use of a good tube line stage since you are also running tape.

Thank you all for your input!  Here’s what I’ve gathered:   modern tube preamps will not sacrifice my newfound detail (including Conrad Johnson’s PV-10al), a tube preamp will give me back those rich odd-order harmonics I have a taste for, and use a DAC.

Even Paul McGowen thinks preamps are necessary:
https://www.youtube.com/watch?v=Dh27E7YKN9s

And, I if I were you, I'd consider one of the Atma preamps.
B

“...a tube preamp will give me back those rich odd-order harmonics I have a taste for...”
I think you meant even order harmonics and the CJ PV10AL delivers plenty of them, maybe a tad too much depending on your taste. Real juicy midrange if that's what are after but a bit rounded off on top.