miniDSP UMIK-1 USB microphone and Mitch Barnett’s “Accurate Sound Reproduction Using DSP eBook” on Amazon.
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They mold to the shape of your ear canal over time to make sure you get a perfect seal with your IEMS.
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But did they improve the SQ? 
What a question?! Of course it does! Man, it looks now like a proper snake-oil cable. Suddenly I can hear in bat territory. 
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But to be serious…
The best $40 I spent was very recently on this…Douk Audio Stereo Line Pre-Amplifier Level Booster for MP3, iPod, iPad, iPhone | eBay
I used this between the analog output of a JVC 3 head cassette tape deck and the analog input of my ML585.
The reason being that as the tape deck had only a 0.6v max output I was having to really crank the volume on the amp and it still seemed wooly in SQ.
This little gadget not only boosted the output to a CD matching 2v ( or more if needed) but fleshed out the SQ in spades.
It uses an opamp and my guess is the one used really matched up well with the tape deck and I could not be happier with the results.
I am now revisiting a lot of my tapes!