Passive tube stage/non-DAC device?

Check out the Musical Fidelity X-10D and the Taddeo Digital Antidote I and II. Happy listening.

I just reread your post and realized your were looking for tube devices. The Taddeo units are not tube devices, but effective.

There is also the Z-man Analog Signal Enhancer (ASE) which uses a single 12AX7, has a decent power supply (not a wall-wart like the MF X10-D) and an on/off switch. I've used mine in some high-res systems with very good results. Currently I use one in a system with Magnepan MG-12s and an InnerSound ESL amp. They can sometimes be found used for $90 to $120, but I don't think they are currently in production.