Best preamp for and EAR890 / ESL57 rig

Jonathan, Why would you recommend removing crossover and subs? The main drawback of the Quads is that they can't go below 55Hz. The Gradient woofers were designed specifically for the Quads (well, I could not find the ones designed for the 57. So, I am using those designed for the ESL 63) and allow you to enjoy the best of the Quads (midrange) and a pretty tight bass from 120Hz to 22Hz. Giovanni

Giovanni, I see that your subwoofer's crossover goes between your preamp and the power amps. It has a low pass output below 110 Hz and a high pass output for the main speakers (from the Gradient page).
So, I was wrong about paralelled amps loading down the preamp down to 9K...and Y adapters.
Anyway, I suggest that you ask Gradient or the distributor what is the crossover's input impedance.
If it is lower than 20 K ohms, most tube preamps will not drive it properly. In that case you need to look for a tube preamp with very low output impedance, lower than 500 ohms.
Any decent tube preamp will work wonderfully with a 50K load, but very few match well with a 10K load, typical of solid state devices.
Preamps with "super tubes" do have low output impedance: BAT, ARC come to mind.
The EAR 868 may work, if it is transformer coupled design as the EAR 912. Ask EAR about output impedance.
You will have decent bass if the crossover input impedance divided by the preamp output impedance equals 10 or more.
I suggest a factor of 20 because most tube preamps are limited in the bass by the size of their output coupling caps. The manufacturer may quote (for example) 500 ohms output impedance at 1 Khz, but it is much higher in the bass range, because the output cap's reactance increases at low frequencies.
Good luck

Thanks casouza. The power amp i use for the woofers actually has a 20K input impedance. I will ask Gradient about input impedance. Thanks. Giovanni

Giovanni. I took delivery of an 868 line only about a month ago. I took a chance and bought it on Bob Levi's high recommendation. I love it. It's dead quiet and more musical than my previous ARC Ref. 3.

Output impedance is 600 ohm. It's a balanced design, transformer coupled in and out with de Paravincini's famous transformers. Same circuit as the 912 but without the extras. I wouldn't be supprised if it sounds better than the 912 since less in the circuit. Only 2 tubes to worry about and the 7dj8's are cheaper than the 6dj8/6922 tubes. If you already have some nice NOS 6dj8's, you can use them also. I'm using a pair of 1957 Philips Holland pinched waist 7dj8's. When I have some time, I'll experiment with other tubes but for now I'm not inclinded to. Looks great with the chrome faceplate and knobs.

Caveats: Cheap plastic remote for volume only and no volume readout. But hey, for the money I'm not complaining.

I'm running a playback Designs MPS-5 into the 868 and then into Lamm 1.2 REF. mono's.
Frank

Giovanni,

My preference with Quad 57's is without any subwoofer or crossover. This makes things technically more extended but in my opinion ruins the magic of the original Quad. Balance is the key to a great system and the Quad by itself has balance. The subwoofer ruins this balance. Just my opinion and from years of listening to 57's. Don't focus on the frequency response specs, just listen to the music. The Quad will be much more connective without all the extra gear. I have Entec subs that I tried with my Quads. And Decca ribbon tweeters. All a waste of time and energy in the long haul. I no longer use Quads but did so for many years.
The impedance specs for the preamp are misleading. Things are not so cut and dry as 10X or 20X. Try and see.

Jonathan