I just ordered a pair of Zu Omen Def's

I have owned the Omen Defs and paired them with Decware and it is a great combination. I auditioned many more expensive amps in my system and the Decware Torii and Mini Torii were two of my favorites with the Omen Defs. The other two were the Pass XA 30.5 and the Art Audio PX-25. The Zu's paired with the Mini Torii were so correct tone and timbre wise. The XA 30.5 brought amazing speed and clarity and powerful bass and the Torii was a middle ground for the two. The Torii brings a lot more bass than the Mini Torii but loses a little (not much) on the Set magical midrange. However the Torii is massively revealing of any weaknesses in your system or in your room acoustics. The Mini Torii and the new SE34.3 are much more forgiving.

BtW, if you like to modify, then you absolutely should upgrade the caps in the Omen Defs to Mundorf SIO.

I'm getting the amp bug now. The Omen Defs have been sounding outstanding with my pioneer elite receiver. From what I've been reading, a much higher quality amp will bring lots more depth into the soundstage. The Pass Labs amps seem to be a good SS fit (I'm so afraid of losing any bass by going with tubes).

Would older, used Pass amps provide a good fit?

You have endless amp options and yes, older Pass amps are fantastic with Zu. I believe the First Watt F3 would be a good choice. A more expensive choice, but one that is great is a XA 30.5 or the new J2.

As for tubes, there are many choices that will give you great bass. There is a Mystere amp for sale right now that would work very well with your speakers. Another great choice would be a Decware Torii or a Melody amp.

Good luck on your choices.

Do you still own them ?