They get progressively better. That said, the Tempo can and does compete sonically with amps that are in the $10-$15K range. I say that without a hair of hesitation.
As you rise in quality, the amps progressively get better. The move from Tempo to Audion is noticeable as you are moving to two separate PSs so any risk of crosstalk is gone. As you rise further, parts, filtering, etc... gets better. But it is evolution, not revolution. It is obvious they are from the same company and if you listen to the Tempo vs the other amps you would say they are progressively a little better. It never works that amps that are 3x more expensive are 3x better.
Vs external competitors, I would tell you that each amp is fairly priced in terms of the quality it delivers. Vs other AGD amps it is tough because the Tempo sets a high bar for their entry level amp.
Eventually I will have one of each of these (tempo, audion, gran vivace, alto and andante) to loan out. I need to sell enough to justify that investment and the Tempo represents the line quite well.