(Rightly or wrongly, I thought that a continuation of a months-old thread would only be seen by those who had originally responded to it.)
There are indeed hundreds of music player apps, some with an EQ function. But the sensitivity of my headphones is such that I have to have the volume near-full, which gives zero headroom for a 12dB boost at 3kHz, which I now enjoy. Also, having (largely) tamed iTunes and the standard Apple music player, the others I tried didn’t seem capable of managing my music and podcasts how I like.
Before starting this project I tried both a telecoil neckloop and direct input shoes. Both gave a bass roll-off starting at around 500Hz, dropping to -15dB at 250Hz and -40dB at 125Hz. I’m not a thumpy bass person but that was just terrible. I couldn’t definitely say whether it was the HAs (Oticon Spirit Synergy) rolling off (my audiologist said not) or the vented domes that just don’t couple the lower frequencies. (I tried larger domes, which were no better and less comfortable, and also the triple-flange earbuds from my Etymotic ear canal earphones, which are fine with the earphones they belong to.)