Best cost no object tube phono

What about adding subs to the Quad set up ? Would they integrate well ?

@inna-- been there. I've had this pair since the mid-70s, set up at one point as a mini-HQD system. I boxed them in 1990 when I bought a pair of Crosby-Quads. I promised myself that at some point I would restore the 57s and when I moved, I had the opportunity. They got shipped off to Electrostatic Solutions, and when he finished his work, they were shipped to me in Texas. I put them back on the stock feet. I'm good with them- this is how I first used them back in the '70s, and learned to listen through this speaker. It's cool to have owned gear for almost 50 years that is still delivering the goods. Not a speaker meant for all purposes, but for what it does well, it's a classic. If I want to go big, I use the bigger system upstairs. That thing can now deliver serious bass, coupled with the horns mid and top and I'm happy. 
My experience in the past was not satisfactory in adding woofers. I suspect Maggie bass panels might work, I know some have advocated open baffle designs, or other types of bass augmentation. The old Quad can give you the impression of big bass given the size of the panel, but it isn't terribly deep and won't play loud. It isn't meant for Sabbath, but it's great for jazz, voice, folk, etc. 

I see. They just want to play the way they were designed to play.
Your horns are Lamm tube powered but subs are SS powered, I understand. How would it all sound if subs were driven by tubes as well, I wonder ? And how many tubes would it take?

Dear @whart  : This was my mix-up, sorry:
"   may eventually change out my Avantgarde Duos "

My friend Guillermo has in a second room  ( like you ) the 57s and yes .the mid-range  is something to listen it, unfortunatelly both extreme frequency ranges are not at the same level. My friend uses an extrenal tweeter/supertweeter.

R.

Thomas Mayer (vinylsavor.com) has 3 models single ended tube LCR (10K, 14k and 23K euros full silver) and 2 models fully differential LCR (28K and 75K euros full silver) phono preamplifier.  Each are custom build to order.  All his LCR transformers are Lundahl and Lundahl silver.  I heard the his entry level once while I was in France quite some time ago, and could not compare it to anything then.