This statement is false on two counts.
External crossovers don't contribute noise to any system nor do they add distortion. If either were to happen, it would point to a severe flaw in the amplifier driving the crossover.
Passive crossovers of themselves cannot cause noise or distortion.
Well we will see next week. Do you own an audio analyzer?
Hp 8903B Old but it does the job.
Clearly some ’tube guys’ do have analyzers on hand...
That old junker maxes out at -80dB THD+N. Useless for modern audio gear engineering. Especially for class D design work. No wonder you didn’t know Johnson noise existed. With my analyzer cheap dummy loads have a -92dB noise floor. But your analyzer wouldn’t even pick that up.