These three appear pretty similar power-wise, with the most costly PM6004 probably being the best. Funny how it often works out that way. You can also see that the weight of the units increases with price. This may or may not reflect the robustness of the power supplies in each unit. The other two don't give up much though. You really need to decrease watts by half, or increase to double in order to have a really noticeable difference in power when comparing amps. The Marantz units do have a phono section if that is a consideration for the future. Any difference in sound between these three choices would be more of a personal preference than anything. I am biased toward NAD, but the Marantz amps are nice as well. I've heard several of them in the not too distant past, and my general impression is that they might be a little smoother sounding, but I'm not 100% sure. And smoother sounding is not always better sounding, so there you go.

btw- I have never considered what type of music a system is used for when selecting components for a system. There is enough variety in even my modest music collection, that accuracy (whatever that means) and/or neutrality are the most common things I look for when selecting any component.

I don't think you will go wrong with the Marantz PM6004.

It looks like a good choice.

Another unit I would suggest that you consider is the Harman-Kardon 3490; it is a very good unit, and might be slightly less money. The UK Harmon-Kardon webite sells it for 400 pounds. It has considerably more power and additional features that the 6004 does not have.

Selling the complete Onkyo system should be the best way to get some money out of it.