[c4sK]

Brand new to the audio game and I have just bought myself the DT 1990 Pro and the L30/E30 stack. Will I hear a difference between say Amazon basics (they have some good reviews despite the price) and some more expensive RCA's? I'm based in the UK and only need 20cm. Cables4All have 20cm but it would be more convenient to get them from Amazon as I have other stuff in my basket and I get free delivery.

 

[ThanatosVI]

Differences can be heard, usually that is only worth it as finishing Touch for high end Setups.

In your case Amazon basics is totally fine.

 

[Speedskater]

with 15 meter (50 foot) long RCA interconnects there will be differences in background noise levels. With 1 meter RCA interconnects, it's doubtful.
But for interference problems, length isn't the big factor and the cheap ones might do better.