Falling out of love with tubes

I also have an SA-250 and I agree (in my opinion) they are a great sounding amp.

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To audibleaudio - Just to add to my posting about the SA-250 being a good sounding amp.  When I compared at the time to Bryston 3B-NRB and McCormack DNA 0.5 with my associated equipment and in my listening room, the SA-250 held its own or was better.  I would suggest as you mentioned getting a pre-amp however. Also regarding the statement you mentioned about hearing detail you never heard before, this does not surprise me. When I was younger and just started experimenting with equipment I had some old Heathkit monoblock tube amps and a Heathkit tube Preamp although I don't remember the model numbers. This old equipment was hailed as being very smooth and pleasing to the ear and it sounded great to me at the time. Then I listened to a song (on album of course) on my fathers system which consisted of an Audio Research SP3A Preamp and a Dual 75 Amp.  This was in the early 1970's. I heard detail and sounds in the song I never even heard before. Then I began to understand. Although a lot of the old tube amps (Eico, Heathkit, Dynaco) are very sought after, they are pleasing to listen to, but far from state-of-the art. I still appreciate them, but I can hear the difference even though I am getting old now and my ears are not so great any more after working around industrial machinery so long. Anyway good luck in your quest. 

" 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.