You do know what the E30 sounds, you just don't know that you've heard it
To refresh, you'd heard it with the Quad S5s and then the Boston Acoustics A26 while they were being driven by the SMSL DA9.
Of course, the audition was entirely inadequate to judge it's Sonic prowess since all the components in the chain were being experienced for the first time
Having said that, i do have the Zen DAC (v1) and the E30 and my experience with the two products resonates with yours vis-a-vis the E50 vs Zen Signature.
The Zen series has a very well balanced and wholesome presentation which may not stand out in any particular regard but that is in fact it's USP. It sounds just right and never fails to please no matter what music or gear you throw at it (and i have indeed tried it with a variety of gear).