The PM500n and the PM600n have the same power, but the PM600n has a better power supply and better components. But the difference in price is substantial, and the available PM6003 is diminishing all the time, though the prices on the older models is attractive.
Can you give us a working BUDGET? That might allow us to suggest other amps that would work.
If you are looking at the PM6004, that implies you have up to £300 to spend. There are several might impressive amp just under £300. Some with substantially more power than the Marantz.
The older PM6003 seem to be running about £220 to £250, and are probably a good bargain, and I wouldn't expect there to be that much difference between the PM6004 and the older PM6003.
The Rega RS1 have a fairly high Sensitivity of ~90db, and an 80 watt power rating. It should not really take a beast of an amp to drive them.
You also want to consider the features you need. It seems you need a Sub Out. Do you need a PHONO input? Do you want or need Tone Controls?
And of course, a direct clear statement of the working budget.
You've got speakers that cost nearly £500/pr. Yet you are looking at amp as low as £170. Though admittedly, you are considering amps as high as £300. So, again, we need to get the working budget cleared up.
Another amp worth considering is the Yamaha AS500 with 85w/ch, Sub out, Phono In, and selling for about £239.
http://www.google.co.uk/search?hl=en&tbm=shop&q=Yamaha+AS500&oq=Yamaha+AS500&gs_l=products-cc.3..0l2.5102.9274.0.9634.14.7.1.6.7.0.156.723.3j4.7.0...0.0...1ac.1.whHNjQ5QRU8
If you want an ultra high feature set, with tons of power, consider the Harman Kardon HK3490, AM/FM, 120w/ch, High Current Power Supply, DAC with Optical/Coaxial inputs, Sub out, Phono In, Pre-Amp out, Main Amp In, etc... all for £279.
http://www.google.co.uk/search?hl=en&tbm=shop&q=Harman+Kardon+HK3490&oq=Harman+Kardon+HK3490&gs_l=products-cc.3..0i10.1887.7741.0.8104.20.10.0.10.10.0.141.1030.3j7.10.0...0.0...1ac.1.s_PrjutPq6Y
There is the Onkyo A9377 digital amp with 80w/ch for £245 -
http://www.google.co.uk/search?hl=en&tbm=shop&q=Onkyo+A9377&oq=Onkyo+A9377&gs_l=products-cc.3...1449.10834.0.10969.15.8.2.5.5.0.111.799.4j4.8.0...0.0...1ac.1.e4F0K-2tSDs
The NAD C326BEE with 50w/ch at about £305 -
http://www.google.co.uk/search?hl=en&tbm=shop&q=NAD+C326Bee&oq=NAD+C326Bee&gs_l=products-cc.3..0.1564.5947.0.6443.11.7.0.4.4.0.107.715.1j6.7.0...0.0...1ac.1.Pc8nJiwdc8A
Just a few thoughts.
Steve/bluewizard