Head units vary in the output between models, brands.(2x35W etc..)

Is it right that.. As long as you have a good clean sound of a certain minimum power
coming from the HU, then your amp will determine the final output?

Or is it a case of the more you put in the more you get out?

depends on how you wire up your amp.

In normal cases you would connect the LINE OUT's from your HU to your amp. In this instance, the internal amplifier in the head unit is disabled, so it doesn't matter whether you have a 30x4 output or a 60x4 output, since it is bypassed.

However for those people who are poor, and have headunits with no LINE OUT's, then they would need to wire up an amp (if they have one) from the speaker terminals of the HU to the higher loading inputs on the amp (not all have these). In this case, I would think the the more you put out from the head unit, the louder it will be. Your amp will just increase that input of yours by whatever level you have set it to.

Using line level convertors with a head unit of greater amp'ed output is not going to give you more output power from the amp. The LLC converts the head unit's signal to a lower voltage for the amp. This kind of setup should be avoided (or using the high level inputs on the amp) as you introduce a lot of distortion.