I use a Triode TRV P:845SE / a single 845 per channel operated SET and good for about 22wpc. This has been one of the amps in steady rotation in my system, for the last 6-7yrs. Bias is easily adjusted and has been rock steady. It sounds as good today as it did when I purchased it. I say all of this because SET amps and old tube amps in general have reputations for requiring periodic maintenance.
Many recommendations for the venerable ST70 / AKA the Model T of tube amps ! They are ubiquitous and easily found in various degrees of operation and condition. It should be required education for those new to tubes to listen to a fully restored ST70 (stock specification- no modifications) before moving on to more expensive amplifiers. It sounds smooth and its sins are of omission.
As noted the ST35 is a superior amplifier; more liquid, incisive, detailed but with plenty of tube warmth. The transformers are outstanding and among the best produced back in the day. They other key to its sound is the use of 6bq5 output tubes. This tube has a well earned reputation for sounding wonderful in many applications. Some say it is the ideal tube for original QUAD ESL speakers, and it easily drives horn speakers into sonic bliss. The ST35 is a great place to start. Others to consider are Eico HF14 / 86, Heath UA-1 and AAA-111, Fisher 20a and 30a monoblocks. Simple circuits and easy to restore, but great output transformers !
Regarding SET amps, if the ST70 is the most common amplifier, a close 2nd would be a 300b SET amp from any 1 of several manufacturers. The basic circuit is very simple thus price differences come down to transformer quality and design of the power supply. If you want to investigate 300b amps, there are kits, but I would look for a Triode TRV, Line Magnetic or Cary 300b amp. All sound great, are easy to use with many speakers, and are built to last a long time.
Good Luck.