Amp Upgrade time, $6000 budget, used or new.

If you care about soundstage you may consider VTL MB450 used.

https://www.audiogon.com/listings/lisa2afe-vtl-mb450-signature-monoblock-amps-tube

https://www.stereophile.com/content/vtl-mb-450-series-iii-signature-monoblock-power-amplifier-page-2

I had been using only tube power amp except driving subwoofer for last 20 years.

I had been happy with Line Magnetic 508 for last 3 years but it may not have enough power to drive your speaker.

There’s a pair of McCormack DNA1 monoblocks with Steve’s Ultra Platinum upgrades for $3500 on US AudioMart.  Pretty darn close to perfect IMHO. 

@soix McCormack is some mighty fine gear, serviceable, reliable, 
mable to call,and talk with the Steve himself!

 But he Sanders Magtech is beyond even Steve’s amps.

mplease call, talk, with Roger Sanders, he will,blow your mind.

there is really no amp which will give you the headroom, sound, and longevity of the Magtech!!

look into it!!

 You will thank me in the long run!

Soundstage = Ayre :-) but lots of very very fine amps mentioned 

When I spend that kind of dough, I won’t do it without a home audition and right of return. That lets out most used gear. I am using Janszen hybrid electrostatics, and I don't want a harsh amp to take away from their beauty.

If you can take the copious heat, a Pass Labs X250.8 would meet your sound profile. Pass recommended (to me) the X series as better suited than the XA for electrostats. Reno HiFi often has demo units at about 20% above your price point, and they will take it back if you don’t like it.

I auditioned that amp, enjoyed the slightly lush and very atmospheric sound, and sent it back because the heat was too much for my small room. As @asa said, Pass Labs amps are widely loved and hold value.

I auditioned the Magtech, too, which to my ears had a chillier sound profile. (I know many hear perfection in it; and I agree Mr Sanders is a genius and great to do business with.) I decided on a Bryston 4B3, with sound somewhere in between the two. I am very happy with that, so I should include it in my list, though it has less snob appeal than Pass. Bryston these days has a far less up-front sound than previously.