Some audiophile friends and sales people are of the opinion that "older amplifiers are outdated' and therefore cannot compete with the better new high end amps
Is it true that the best newer amps always sound better than the best older units?No. That depends on which new amp as opposed to which older amp. Not all new amps are better.
Does either technology, better parts, or improved circuitry and engineering make such progress inevitable?If you get rid of 'either' in that question the answer is 'yes'.