@pgastone I’m a huge fan of SET amps and Avantgarde. What a wonderful direction you’re going in. While 300B are amongst my favorite tube type, I agree with others that speed and resolution are equally important to the bloom that you seek.
In this type of application, my favorite amps are custom built by an engineer out of Brooklyn named Oliver Sayes. I have a Korneff 45 clone from him that uses Hashimoto transformers and upgraded capacitors with Russian PIO, Audience Auricap XO and Mundorf. The amp uses a tube compliment of a pair of EML 45 Mesh, a single 6SN7 where I employed a Shuguang Black Treasure (the only place I’ve ever permanently lived with this tube), and a Philips 5R4GYS rectifier. All in all this build and configuration with tubes can probably be had for less that $3500, with tubes costing almost half that.
While I didn’t own Avantgarde, I did use this amp with Oris 200 horns using AER drivers and they were lovely. I unfortunately no longer have the horns as we had to repurpose the room for that system for something else, but I still do own the amplifier and now use it as a headphone amp to drive my Audeze LCD-4, and it is killer!