Can anyone explain to me why a low powered SET would be more "quiet" and work better with such speakers than an ARC Ref 75 or VAC 200iq (as examples)?
@bobbydd Yes. The dealer is incorrect; PP amps are not noisier. But, and this is a big one, that depends on how much gain the amp has.
SETs tend to not have a lot of gain, because they don't make much power and are intended for use on high efficiency speakers. PP amps usually make more power and so usually are on less efficient speakers- and so are designed with more gain.
The gain differences described allow preamps to be more or less universal. A typical line stage might have 10-15dB of gain; if you properly match the amp with the speaker you can use the same preamp with a high power amp with less efficient speakers or a low power amp with speakers of high efficiency and have the same noise floor and use the volume control the same way.
If you combine a high power amp that likely will have 30dB of gain with a high efficiency speaker, you'll be battling a noise floor of hiss, hum and buzz.
So SETs are lower noise simply because they usually have less gain and for no other reason. Obviously, if you have a lower powered PP amp with the same gain, it can be perfectly quiet too.
You can get PP amps to be silent on horns. But you might have to jump through some hoops to make it happen and that might be a problem with larger ARC stuff.
We've been making PP tube amps that compete easily with any SET for musicality, but our amps are unusual in two ways that have to do with horns. First, they have no output transformer (and are otherwise triode and class A; hence the musicality). Second, we make a little jumper plug that allows the user to remove one of the driver tubes and replace it with the jumper, which drops the gain about 12dB without changing its output power. This helps them be quiet with horns and still easily driven by any preamp. I don't think ARC has anything like that, so whatever their noise floor is, about the only way you can improve on it would be to seek really low noise driver tubes for it.