Epanorama

Here is some info you will find interesting and give you all the background info you will need. Good Luck.

As far as the transformer and power supply go, there is no problem with 50 Hz or 60 Hz operation.

However, some manufacturers like KRELL, build in a Freq SENSING stage, that will Cut off power when it detects the 'other' freq. This is to avoid Equipment bought in the US market ( invariably lower price ) and sold in other markets, where the same stuff is sold at almost double the US price....

I don’t think Marantz builds in such a mechanism.

You should have no problem.

Even better, open the SR-5600 and check if there are internal jumpers for 230 VAC operation. Better to use the internal jumpers ( IF they are there, on the Transformer primary ) than an external transfo.

Besides the cost of an External Transfo... the external transfo ill conscrict the sound / dynamics to atleast some extent.

A lot of equipment have jumpers on the power supply board to allow operation on either voltage. Not at the same time of course. Most manufacturers try to make their products as universal as possible. Either voltage, either frequency.
The main problem when operating with reduced frequency is increased transformer temps. Usually not a problem. If the power supply board does have 110/220 v selection that would be a good indication that the supply will tolerate 50Hz.
The external transformer should work fine as long as it is rated for the load plus a little extra. 150% of the load maybe. If you could configure the power supply for 220V operation though that would be a better choice.