Class D amplifier with TPA 3250 board

Do you hear hiss when the amp is hooked to the speakers, turned on, with NO source (including preamp) connected?

While sometimes hiss is connected to an amp that simply doesn't have a very good S/N ratio, it is just as often connected with a mismatch between input sensitivity and gain levels between sources, preamps and the amp.

If you have no hiss while just using the amp only, add components back one at a time and see when hiss returns. You might be able to fix the problem by decreasing the input sensitivity of the amp, reducing the output of the source, or using resistors in between the two to reduce gain.

One other thought -- have you tried different tubes in your amp? I know nothing about the design of your amp, but if the hiss is equal in both channels, it may be a bad preamp tube. Ones like 12AX7, for example, are dual-channel tubes so one tube can affect both channels.  Bad driver or output tubes typically affect only one channel unless the whole complement of tubes is getting old.

Class A/B has nothing to do with noise -- it simply describes how the amp is biased -- class A mode at low power, switching to class B operation as the power increases.  That switch can take place at a fraction of a watt to many watts, but doesn't generate or eliminate noise by itself.

I've not used either specific amp, but do know that Yamaha is a good company if you like their sonic character. I'm not familar with Atoll.

Finally,I doubt that a power conditioner would affect the hiss at all -- hiss is not the typical way in which power line noise affects sonic playback.

Confusing thread -- first hiss, now hum! Two distinctly different problems!