Are audio, recording studio, military signal transformers etc the best investment on the planet?

The Gardners lower bandwidth series MU7513 mic or line to grid transformer sold on eBay.co.uk for about £80 on 18 August 2019
eBay item no: 123871427096


Bidding was missed.


But the rare GT.5 Gardners Audio Frequency Transformers catalogues had been bought on eBay.
And the kind seller included price list documentation and a 31st October 1972 vintage letter from the factory to the person interested.


Price for MU7513 on Sptember 1972 is:
£2.97


Can't imagine what would today's price be if so many university textbooks did not speak low on them suggesting opamp energy consuming alternatives with capacitor feedback to simulate an inductor etc or active balanced stages etc.


Thanks to academics price is kept low.


The MU7513 (+ or -) 0.5dB bandwidth is published 50Hz to 10KHz.


So why?


Perhaps one reason is exactly this fact.


They reduce bass or treble heaviness due to Fletcher Munson, Voice effort effects etc so that singing voice sounds with flat frequency response not from mic to loudspeaker but much more importantly from singer's vocal chords to listener's brain. [Lowe, Morgan]


Reference:


Sound Picture Recording and Reprosucing Characteristics - D. P. Lowe, K. F. Morgan - Journal of the Society of Motion Picture Engineers