Another great match, which I highly recommend, with fantastic synergy for the RU6 is the one that comes out with the Etymotic ER4SR. Since it wants iems as neutral as possible, I thought... which is the most transparent and natural iem I have? Looking well in the iem drawer, the ER4SR came out, simply the most natural iem that exists in the world and still today unmatched for its reference sound. I was expecting a certain synergy, but I didn't imagine what I would hear after a while... a deep and powerful bass (for which they have always been criticized because they are extremely flat) like I had never heard on the Etys. Only the RU6 could make me look for the ER4SRs after all this time... but not because they sound bad... it's just having to get used to inserting them again, let's say, not really natural... lol
If you want to really hear the RU6's voice you have to use a stethoscope, and I did. Great!!!