This is an easy one, in fact. Try to compare tubes when the difference is subtle.
I also once had fun comparing RCA cables that sounded very very similar and very close to each other in my system. Both were absolutely excellent, and the price difference used was $1000.00. Good for me that I needed them both. The more expensive one was a little more neutral and balanced, and the less expensive one was a bit more vivid and colorful, but not colored.
The less expensive cable was Gabriel Gold Infusion 2.
The more expensive one was Wywires Diamond.
I compared with analogue source, both phono stage and cassette deck, from the source to preamp.