There is no "optimum" with Eros, there are only minimums. This is because there are no inductances for the cap to resonate with.
The primary guidance is that it must have enough capacitance to pass bass frequencies to the following preamp or integrated amp. I usually find that a -3dB point of around 5Hz is more than enough. If the Eros drives a 100K input resistance, then 0.33uF would be enough - but with a 25K load, you'd want 1.3uF. Into a 15K load, which is the minimum recommended load, you'd want 2.2uF. But at 1uF, it's still only 10Hz, and may not be audibly different depending on your system.
If I recall correctly, the original design called for 1.0uF but at some time that cap was on back order so we shipped the 1.5uF cap from the S.E.X. amp. It sounds equally good, and reduced the number of parts in inventory, so there has been little pressure to revert.
