I recently found a really nice Klipsch center channel which I'd like to use as a back rear to finally try out some Dolby EX, and DTS ES material. The only problem is that my aging receiver isn't capable for this and I have no idea which if any nowadays will do this format since it was quickly abandoned for 7.1 and above.

Plus I'd really like to get it hooked up and try out true 4 channel Dolby Stereo since my reciever currently spreads the rear mono surround across both L and R surrounds.

Any suggestions perhaps on an inexpensive older receiver that can do these formats?

Any reciever with es/ex support will give you the option for single rear center in the menu. 4.0 mono surround isn't supported by any consumer processor I know of.

I used the onkyo tx-sr600e for 9 years. I was really happy about that one. It lacked a phono input but if this doesn't bother you it would be a great purchase for 5.1 or 6.1 discrete audio
Dts es dolby 5.1 ex it all sounds

Phono input equiped would be the tx-sr700e

Sold it two years ago to "upgrade"
I'm outdated once again by dolby atmos i'm afraid

I'm sure there are many other brands that would prove to be solid performers for extended surround.

Using a marantz sr-7007 these days for the obvious hd formats and hdmi thing.
Running the same 6.1 b&w speakers . Never felt the need for two subs or 2 surround back speakers

Substance is correct. Any decent modern receiver will give you the option and flexibility of selecting your speaker setup, whether it be 5.1, 6.1. 7.1 etc...of course depending on the model.