Best cost no object tube phono

Dear @whart  : Normally planar speakers are not high sensitivity and are far away from your horns.

Marbe you can make some research to find out if a planar small speaker can works for you and the Lamm amplifiers working as satellite speakers with your subs handling frequencies from 90hz-100hz and lower. In this way the Lammas will " see " frequencies over 100hz and with out the low bass range you could have headroom to handle the planar speakers.

Other alternative could be the Kerr K300 speakers that comes with a planar tweeter crossing at 1.95khz that means it handle a wide frequency range and along that the K300 is a transmision line design that's has really low distortions neare the performance of planar speakers. So that  you can get that immediacy  and take advantage too of those subs that will handled the frequency below 80-90hz, sensitivity of the K300 is 89db.

Btw, your Avantgarde are very fine speakers, the distributor here ( not any more. ) was a friend of mine and I had several sessions with because not only in his demo rooms but at his home he has those Avantgarde in his system. very nice.

R.

@rauliruegas - sorry to have confused things. I am not equating the electrostatic with the horn in any way except midrange clarity. Impedance, amp demands, etc. all different. I do run my original Quad 57s, which I have owned since around 1973-4 in another system in another room, they were restored, along with a pair of Quad II amps that date from 1961, also restored with primo glass in them. Totally different character, but the midrange on the Quad-- oohla,la. And the horn gives me the clarity, with dynamics, and by adding big subs to the woofer system of the Duo, I get bass impact. The thing that really gelled with these was a switch in cartridges from the Airtight or Lyra to Koetsu stone bodies-- bass with body. Who knew?

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?