Best amp for Avalon

Radian HC means "High Current".
Specially built for the Spectral amps.
TRL S100 monos will be perfect for mid and high.
Not for the 2 power hungry bass drivers in sealed box config.

Johnsonwu is correct. I recall it being a special package with the Spectral amps and special MIT cables and a 3D moniker. A class A amp like one of the offerings from Pass would be a good start. I also noticed an Audia Flight 100 listed for 3500.00 which is an excellent price for 100W of class A power.

Do you have to use identical amps if you biamp? Can you use tube on top and solid star on the bass. What about gain difference?

Most Avalon owners would have to use an RCA to RCA 100k trimpot right hat the input jack of the bass amp which usually has higher gain to attentuate the bass.

I have owned Ascent II, Eidolon, and Monitor and have tried more than 10 amps on the Ascent IIs.
Radian is of similar drivability so be prepared.
The sealed box Eton woofers are almost impossible to drive.

High power tube amp will make the highs grainy.
Low power and it clips very easily.
The only 2 amps that were able to drive my Ascents full range non bi-amp to play at decent volume without heating up the whole house, ironically, were solid state... TRL Samson and Pass 350.5.
You can forget about those Audio Research monos, Krells, and Levinson.

Johnsonwu
Did you go all solid state or tube preamp. Which amp did you like better Pass or TRL