Who are your favorite custom amp builders for low powered triode amps?

Yes.  Triode is nice. They use quality iron in all of their amps 

Triode labs-Finale Audio out of Toronto makes some very nice amps for reasonable prices. I've owned three now over the years and can't recommend them fast enough for the efficient speaker groups. 

I have not heard Aric Audio or Don Sachs amplifiers, but, just what I've seen on their websites they do appear to be interesting builders.

There are a number of small builders that make good gear, e.g., Atmasphere, Lector, Synthesis, Shindo, but I don't know how much customization is involved.  The builder of my linestage, Aldo D'Urso, normally did not offer remote control of volume, but I wanted it so he built it that way.  I also specified balance control.  I wanted output transformers to match the input transformers on my amp ( I had choice of transformer output, capacitor output or a switchable choice of either).  I had choices on components used in the build; I chose the not-crazy-expensive Western Electric output transformer (I know another buyer who chose $20k output transformers in his linestage and battery power, and other high priced options).  I did not bother to specify anything about the chassis and cosmetics, so I ended up with "ugly," which I actually like.

You never know who will be around tomorrow…. Hopefully most builders and small companies  with a good following survived COVID.   That really hurt small companies that buy electronics in smaller quantities.   Plus a lot of businesses temporarily shut down losing skilled workers.  

The only drawback to buying these simple basic amplifiers from boutique companies is they do not stay in business very long, leaving you with an orphaned product that has poor resale value when you decide to upgrade.  Having said that, I have owned SETs from Sophia Electric for the past 20 years and recommend them.  They carry in house all the parts you might need to retube or modify your electronics and are available for personal consultation.  Great company!

Decware offers the finest triode amps available at a reasonable price. The hitch is that you have to be on a long waiting list unless you buy used, which is a great option. 

I have really liked my Aric Audio Super 300B SET.  

https://aricaudio.com/products.php?product=SPR300BSET

I am also a big fan of Brandon Wade's amps.  I have a custom 2A3 amp from him that I really like.  He has been finalizing the design of an 811 amp that is higher power but still SET.  I heard the prototype and liked it too.

https://apolloaudiosystems.squarespace.com/

I’m a huge fan of Oliver Sayes out of Brooklyn. He makes incredibly affordable DHT designs, cloning and building off Korneff 45 and Yamamoto designs, amongst many others. He can source top notch transformers and components, either ones he may recommend or ones you request. I sold my last one to a person here on Audiogon. It was a very special amp, but I no longer own high efficiency speakers. Feel free to PM me if you want me to put you in touch with him. 

It will look similar to this preamp.   Based on the 205d tube 

I love my Toolshed Amps Transcendence 300b.   Love it so much that Matt is building me it's companion DHT preamp right now. 

https://www.toolshedamps.com/

Call Aric of Aric Audio.

Sounds so cool. Thanks for sharing. 

The custom builder of my linestage and power amp builds gear based on old Western Electric gear.  He utilizes a lot of vintage parts in his builds.  While he has done 300b triode amps, most of his builds are low-powered pentode amps.  His name is Aldo D'Urso and he lives in Italy.  

My pushpull amp is a rebuilt version of a Western Electric 133A amp.  It utilizes original Western Electric input transformers and output transformers and vintage paper in oil caps and vintage resistors.  The power transformer and choke are modern parts.  The input/driver tubes are Western Electric 348A tubes (four of them) and the output tubes are 349A (four of them in a stereo amp).  The use of input transformers mean that the amp is best used with a linestage with complementary output transformers.  This pushpull amp puts a whopping 5.5 wpc.

My linestage utilizes such output transformers as well as input transformers (all Western Electric iron).  The linestage runs Western Electric 310 A and 348A tubes.  It is NOT a Western Electric replica in that it has remote control of volume.