What amp do I need to power my Acoustat 1100

It's a process with numerous SS or Tube amps to choose. You have to audition the Krell and Parasound A21 hopefully with the Acoustats to determine what fits your listening experience.

If your buy new or used what is your budget?

If you want an SS amp that won't freak out when faced with electrostats, consider the one from Roger Sanders (formerly of Innersound) http://www.sanderssoundsystems.com/

If you prefer tubes as I do (for driving electrostats) I recommend one (or possibly two as monoblocks) McIntosh MC275's. The McIntosh Unity Coupled Circuit provides as much current per watt as just about any amplifier made, so it's well suited for driving electrostats.

Any amp stable at one. ohm. These are not easy to rive becauze they do present 1 ohm laods but employ fonick transforms thst friend's melted with an Aragon m0n0 block palladium 1Ks.
Instead try a favorite for low ohm loads the ICE powered H2O brand.

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i wanna spend $2500 or below.

If you want to use SS; find an amp with MOSFET outputs. They will maintain stability at low impedance levels. One can practically arc weld with some MOSFET amps. I used modded Hafler DH-500s with Acoustat Model IIIs(in the early '80's) with great success(was a dealer, back then). I much prefer tubes, but synergism with electrostatics is harder to reach. Here's a suggestion: (http://www.avahifi.com/root/equipment/amplifier/fetvalve_amps.htm) You don't mention your pre, but tubes would be nice there, and would give you the ability to tweak the system's presentation to suit your tastes.