Can't extend network: AE models the problem?

checking out some other messages I searched out here, I suspect I may have the problem figured out, more or less, but need to figure out how to be sure:

I have 5 airport expresses, bought at different times, that make up a home network designed to play music via a pair of speakers attached to each AE, and one main is attached to a cable modem.

This works ok, though I get a lot of dropout as complained about by many others here, not consistent enough to blame my other electronics in the house, although who knows what the downstairs neighbor might have that is interfering.

Hoping to improve the robustness of the system, I tried to set up the AE to extend the network rather than just join it, but got a lot of error messages when airport utility was trying to 'configure main network' to accept an extension via this airport. I did type in MAC addresses for each additional devices into the main (modem connected) AE settings, under 'allow clients', and it seemed to reset with that data accepted/intact. I just couldn't 'extend' via the clients as I added them.

Which AE models are configurable to extend a WDS network? And does it matter which of them is the 'main' and which are the 'relay' devices? And can I interrogate the AE to tell which model they are or do I have to squeeze into their nooks and try to read the very fine print on them?

I really only need one of them to do the 'relay' thing to the printers, and if one of them is compatible with that function and the others were not, it would save a lot of time to know which one it is, or that I have to replace just that one.