Need recommendation for a preamp.

Have sold both on a retail level for years. My advice?...skip em both and have better sound! Even old Audio Research LS2,LS5, LS7, etc, used sound better with good tubes in em! For $500-$750, you could get much better sound than the ones your describing. Also, if you can find that Cary piece that was around $1200 a couple of years back, that would be the best for your money by far! The ones you mentioned are average at best...don't be suckered in by the "Class A" rating on the Adcom...someone got paid trust me.

If it's the Cary SLP-50 referred to above, it is a very nice little preamp. Since you are interested in passive, you might want to give a Placette Passive linestage a try. I think the used price is still around $900, if you can find one. You should be able to audition a used one and recoup your investment if it's not to your liking. The linestage comes with a remote control that does not adversely affect the sound quality and has three inputs and two outputs standard. They also make a RVC, remote volume control, that is a single in/out and is around $700 used. It is the same volume control as in the linestage and also is remote controlled.
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Check out the Eastern Electric MiniMax. I have had Spectral, Hovland, Audible Illusions, passive attenuators, Copland, Audio Research LS1, LS7,...you get the idea. I love the presentation this little gem gives. It's easy for tube rolling. The value this unit gives is simply amazing. Bill O'Connell at the Morningstar Imports has a wealth of knowledge (and a Hovland too) when it comes to tube rolling. I bought mine from Galen Carol Audio in San Antonio. He is an extremely fair person to deal with and a lot of fun to boot. I believe the website is eeaudio.com. Check it out. A lot of fun and fantastic sound for the money.

I have no affiliation with any company mentioned above.