If
THIS is the Karat 40 you have, it is only rated 130W music power. Any older Denon AVR-3X00 can drive them without problem. For 2 channel receiver, the Outlaw and older Harman Kardon models should be able to the job if you can find one.
For a new one at budget price, how about a Yamaha:
https://usa.yamaha.com/products/audio_visual/hifi_components/r-s700/specs.html#product-tabs
or even the R-S500
https://usa.yamaha.com/products/audio_visual/hifi_components/r-s500/index.html
Again, the 4 ohm Karat 40 is only rated 90W DIN, 130W music power. Any good 100WX2 receiver can handle 130W music power even at 4 ohms. If you push them hard listening to highly compressed music, a much more powerful unit may blow them up anyway.
Ideally a 150 to 200W two channel receiver would be good but if a good 100WX2 receiver is hard to find, the more powerful ones are going to be even harder to find and likely costly.
If you don't mind integrated amps, then there are more choices.