The Leach SuperAmp has 100% feedback at DC. This means that a constant DC voltage cannot appear at the output. If you mean turn on pops and clicks, it is never a bad idea to protect against those. My stereo Leach SuperAmp does not incorporate relays and has no DC offset problems.
A new car needs neither a jack nor a spare tire. 10 years down the road you will find out that the tiny lug nut wrench won't remove the now rusted nuts and the now rusted jack won't lift the vehicle and the space saver spare will only get you to the nearest tire store at 20 miles an hour (if you remembered to check the air once a month for the last 10 years). The foldback current limiter in the Leach will protect the amp from short circuits when it is new. A relay will keep it quiet as it ages, and a crowbar will protect the speakers if it blows up. I own a professional power amp that has two stage inrush current limiting, foldback current limiting, a clipping limiter, a speaker relay, and a crowbar. While it costed twice as much as other amps with the same power, is cheaper brothers have been in the landfill for quite a while now. With a few mods it even sounds good! The Leach amps sound great, being a DIY project the other details are up to you.
If I decide to go for protection circuit, would be OK to use a kit K4700 from Velleman. It has a delay and some kind of DC protection with op amp. But could I use this circuit for my dual mono design.( one transformer with separate secondaries for each channel)?
I am in the exact situation as you are. But I've bought two K4700 kits with the intension to leave one channel DC proctect not connected for each kit. I am building the JLH for ESL mono blocks the soft start feature of the kit would have to be in two locations anyway. I don't understand how you can connect it(one kit) to two mono blocks anyway.
Tell me what is the power and voltage rating of relays. I got one small toroid transformer ( 12 Volts. 20VA) to power mains relay and some LED diodes, because I would like to power K4700 kit with this transformer.