Looking for a 300b integrated amplifier kit for my Avantgarde Trio (107db) horn speakers

Welcome to the Audiogon. Why build, might I ask ? Enjoy ! MrD.

Check out Audio Note Kits.

For the enjoyment of building and learning - new hobby for me

I think Audio Note is the most well known.  If you want to go really budget, however, check out the kits from Asia.
I would post this question on a DIY board.  There are a few to choose from. Google DIY threads and see what you get.  I don't build but am jealous of those that do. My favorite non Audiogon-audio threads website is Audio Asylum they have a DIY Asylum as they refer to them as, it may be all tube kit however.

🙏 thank you, this is helpful

In your search for a 300B integrated amp kit I would strongly suggest you consider an Elekit TU-8600. You can find the kit for around $1200.00 without tubes at V.K.Msic or Tube Depot my suggestion would be VK as you get Victor Kung’s help and suggestions as part of the deal . The build is pretty straightforward although I made a stupid/ rookie mistake that Victor trouble shot for me . 
 My own TU8600 is fitted with Gold Lion outputs a Telefunken 12AX7 and HP5963’s in the 12AU7 position. It’s driving Zu Omen MKII’s or LS3/5a’s , and works well with either speaker in my large ( but lively) listening room. The Zu’s with their 97DB efficiency can play quite loudly in my 27’x19’ room while maintaining that exquisite 300B sound. The LS3/5a’s reach more modest levels but the amp really brings out the best in that lovely little monitor. I am familiar with Audio Note kits and believe you’ll find this kit comparable for substantially less money.

Found this site interesting and he has a 300b kit for $1,700. w/o tubes.
No interest in building one though.

https://www.transcendentsound.com

+1 re: AudioNote kit. Also look at Bottlehead Stereomour. Both fit the bill.

You will find wealth of information in DIYAudio web site. From digital sources (DACs, Streamers,
etc) speakers to amplifier (tube, solid state) design.

AN have remote control??